Don't condition neqo_common::log::init on cfg(test)

[larseggert]

          Turns out we cannot make the initialization conditioned on `cfg(test)`, because before a test uses one of the logging macros, the log init won't happen and there will no output :-(

Originally posted by @larseggert in #2291 (comment)

[larseggert]

Test case: RUST_LOG=debug cargo nextest run handshake_mlkem768x25519 --no-capture produces no logging output.

When a qdebug! is added to the handshake_mlkem768x25519 test, it does (and not only from that log statement, but from called functions as well.)