Test coverage gaps found by cargo mutants

[mgeisler]

I ran cargo mutants on the code and it found a few potential things we can look into.

For each of these changes, cargo test passes both before and after, suggesting that we should add more unit tests to catch these changes in behavior:

i18n-report/src/main.rs:91:5: replace all_stats -> anyhow::Result<Vec<MessageStats>> with Ok(vec![])
i18n-helpers/src/lib.rs:821:59: replace && with || in translate_events
i18n-report/src/stats.rs:37:13: replace + with * in MessageStats::total
i18n-helpers/src/bin/mdbook-gettext.rs:38:8: delete ! in preprocess
i18n-helpers/src/bin/mdbook-xgettext.rs:30:5: replace main -> anyhow::Result<()> with Ok(())
i18n-helpers/src/lib.rs:510:49: replace + with - in parse_codeblock
i18n-helpers/src/bin/mdbook-gettext.rs:56:31: replace == with != in main
i18n-report/src/main.rs:166:34: replace == with != in po_files
i18n-helpers/src/lib.rs:429:5: replace is_codeblock_group -> bool with true
i18n-report/src/stats.rs:95:44: replace += with *= in MessageStats::for_catalog
i18n-report/src/main.rs:108:5: replace diff -> anyhow::Result<()> with Ok(())
i18n-helpers/src/preprocessors/gettext.rs:72:9: replace <impl Preprocessor for Gettext>::name -> &str with ""
i18n-report/src/stats.rs:69:20: replace * with / in MessageStats::to_context
i18n-helpers/src/gettext.rs:80:5: replace translate_book with ()
i18n-helpers/src/preprocessors/gettext.rs:41:5: replace get_catalog_path -> anyhow::Result<PathBuf> with Ok(Default::default())
i18n-helpers/src/normalize.rs:225:69: replace - with / in normalize
i18n-helpers/src/normalize.rs:166:75: replace > with < in SourceMap<'a>::extract_messages
i18n-report/src/stats.rs:65:20: replace * with + in MessageStats::to_context
i18n-report/src/stats.rs:65:20: replace * with / in MessageStats::to_context
i18n-report/src/stats.rs:69:55: replace / with % in MessageStats::to_context
i18n-helpers/src/bin/mdbook-gettext.rs:56:5: replace main -> anyhow::Result<()> with Ok(())
i18n-report/src/stats.rs:98:50: replace += with *= in MessageStats::for_catalog
i18n-helpers/src/normalize.rs:16:5: replace parse_source -> Option<(&str, usize)> with Some(("", 1))
i18n-report/src/stats.rs:73:65: replace / with % in MessageStats::to_context
i18n-report/src/stats.rs:109:9: replace <impl Display for MessageStats>::fmt -> fmt::Result with Ok(Default::default())
i18n-helpers/src/normalize.rs:29:5: replace compute_source -> String with "xyzzy".into()
i18n-helpers/src/lib.rs:577:49: replace + with - in parse_codeblock
i18n-report/src/stats.rs:44:9: replace MessageStats::to_context -> BTreeMap<String, Value> with BTreeMap::from_iter([(String::new(), Default::default())])
i18n-helpers/src/preprocessors/gettext.rs:80:9: replace <impl Preprocessor for Gettext>::run -> anyhow::Result<mdbook::book::Book> with Ok(Default::default())
i18n-helpers/src/lib.rs:510:49: replace + with * in parse_codeblock
i18n-report/src/stats.rs:93:50: replace += with *= in MessageStats::for_catalog
i18n-report/src/stats.rs:44:9: replace MessageStats::to_context -> BTreeMap<String, Value> with BTreeMap::from_iter([("xyzzy".into(), Default::default())])
i18n-report/src/stats.rs:37:13: replace + with - in MessageStats::total
i18n-report/src/stats.rs:38:13: replace + with * in MessageStats::total
i18n-report/src/stats.rs:69:20: replace * with + in MessageStats::to_context
i18n-helpers/src/bin/mdbook-i18n-normalize.rs:20:5: replace main -> anyhow::Result<()> with Ok(())
i18n-report/src/stats.rs:93:50: replace += with -= in MessageStats::for_catalog
i18n-report/src/stats.rs:73:65: replace / with * in MessageStats::to_context
i18n-report/src/stats.rs:69:55: replace / with * in MessageStats::to_context
i18n-report/src/main.rs:91:5: replace all_stats -> anyhow::Result<Vec<MessageStats>> with Ok(vec![Default::default()])
i18n-report/src/stats.rs:73:20: replace * with + in MessageStats::to_context
i18n-helpers/src/preprocessors/gettext.rs:29:5: replace should_translate -> bool with false
i18n-report/src/stats.rs:39:13: replace + with - in MessageStats::total
i18n-report/src/stats.rs:65:59: replace / with % in MessageStats::to_context
i18n-report/src/main.rs:159:5: replace po_files -> anyhow::Result<Vec<PathBuf>> with Ok(vec![])
i18n-report/src/stats.rs:77:61: replace / with * in MessageStats::to_context
i18n-report/src/stats.rs:77:61: replace / with % in MessageStats::to_context
i18n-helpers/src/normalize.rs:16:5: replace parse_source -> Option<(&str, usize)> with Some(("", 0))
i18n-helpers/src/normalize.rs:16:5: replace parse_source -> Option<(&str, usize)> with Some(("xyzzy", 1))
i18n-report/src/stats.rs:36:9: replace MessageStats::total -> u32 with 1
i18n-report/src/stats.rs:39:13: replace + with * in MessageStats::total
i18n-report/src/stats.rs:98:50: replace += with -= in MessageStats::for_catalog
i18n-helpers/src/normalize.rs:49:5: replace has_broken_link -> bool with true
mdbook-tera-backend/src/main.rs:13:5: replace main -> anyhow::Result<()> with Ok(())
i18n-helpers/src/lib.rs:577:49: replace + with * in parse_codeblock
i18n-helpers/src/lib.rs:191:9: replace GroupingContext::clear_skip_next_group -> Self with Default::default()
i18n-helpers/src/normalize.rs:32:12: delete ! in compute_source
i18n-report/src/stats.rs:95:44: replace += with -= in MessageStats::for_catalog
i18n-helpers/src/preprocessors/gettext.rs:89:9: replace <impl Preprocessor for Gettext>::supports_renderer -> bool with true
i18n-report/src/stats.rs:77:20: replace * with + in MessageStats::to_context
i18n-helpers/src/preprocessors/gettext.rs:72:9: replace <impl Preprocessor for Gettext>::name -> &str with "xyzzy"
i18n-helpers/src/preprocessors/gettext.rs:89:18: replace != with == in <impl Preprocessor for Gettext>::supports_renderer
i18n-helpers/src/bin/mdbook-gettext.rs:34:5: replace preprocess -> anyhow::Result<()> with Ok(())
i18n-helpers/src/normalize.rs:29:5: replace compute_source -> String with String::new()
i18n-helpers/src/xgettext.rs:327:34: replace == with != in slug
i18n-report/src/stats.rs:100:44: replace += with -= in MessageStats::for_catalog
i18n-helpers/src/preprocessors/gettext.rs:89:9: replace <impl Preprocessor for Gettext>::supports_renderer -> bool with false
i18n-helpers/src/normalize.rs:16:5: replace parse_source -> Option<(&str, usize)> with Some(("xyzzy", 0))
i18n-report/src/stats.rs:38:13: replace + with - in MessageStats::total
i18n-report/src/stats.rs:73:20: replace * with / in MessageStats::to_context
i18n-report/src/stats.rs:77:20: replace * with / in MessageStats::to_context
i18n-report/src/main.rs:159:5: replace po_files -> anyhow::Result<Vec<PathBuf>> with Ok(vec![Default::default()])
i18n-report/src/main.rs:144:34: replace != with == in diff
i18n-report/src/stats.rs:85:9: replace MessageStats::for_catalog -> Self with Default::default()
i18n-helpers/src/preprocessors/gettext.rs:29:5: replace should_translate -> bool with true
i18n-helpers/src/normalize.rs:225:69: replace - with + in normalize
i18n-report/src/stats.rs:44:9: replace MessageStats::to_context -> BTreeMap<String, Value> with BTreeMap::new()
i18n-report/src/stats.rs:100:44: replace += with *= in MessageStats::for_catalog
i18n-report/src/main.rs:33:5: replace main -> anyhow::Result<()> with Ok(())
i18n-report/src/main.rs:113:25: replace == with != in diff
i18n-report/src/stats.rs:65:59: replace / with * in MessageStats::to_context
i18n-report/src/main.rs:73:5: replace report -> anyhow::Result<()> with Ok(())
i18n-helpers/src/lib.rs:570:20: delete ! in parse_codeblock
i18n-report/src/stats.rs:36:9: replace MessageStats::total -> u32 with 0