make test module detection more strict
I started with some small improvements to clippy_utils/src/lib.rs, but then found that our "test" module detection would also catch words containing "test" like e.g. "attestation". So I made this a bit more strict (splitting by `'_'` and checking for `test` or `tests`), adding a test case as I went.
---
*Please write a short comment explaining your change (or "none" for internal only changes)*
changelog: none
a04a7cd9f9