Improve std-lib-regression script (#1360)

Add a dummy proof-harness to it and fix the issue with duplicated lang
items that was triggered when trying to resolve the dependencies. The
issues seemed to be related to the fact that `kani` crate was built with
a different version of the `std` library.

For now, we explicitly add the `kani` crate as a dependency so it gets
rebuilt with the fresh std.

scripts/std-lib-regression.sh

@@ -25,7 +25,7 @@ fi
 
 # Get Kani root
 SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
-KANI_DIR=$SCRIPT_DIR/..
+KANI_DIR=$(dirname "$SCRIPT_DIR")
 
 echo
 echo "Starting Kani codegen for the Rust standard library..."
@@ -39,6 +39,19 @@ fi
 cargo new std_lib_test --lib
 cd std_lib_test
 
+# Add some content to the rust file including an std function that is non-generic.
+echo '
+#[kani::proof]
+fn check_format() {
+    assert!("2021".parse::<u32>().unwrap() == 2021);
+}
+' > src/lib.rs
+
+# Until we add support to this via our bundle, rebuild the kani library too.
+echo "
+kani = {path=\"${KANI_DIR}/library/kani\"}
+" >> Cargo.toml
+
 # Use same nightly toolchain used to build Kani
 cp ${KANI_DIR}/rust-toolchain.toml .