[llvm] [lit] Fix some issues from --per-test-coverage (PR #65242)

[Tobias Hieta via llvm-commits]

tru wrote:

I noticed that the python formatting check failed so I looked into that, the problem is the file `llvm/utils/lit/tests/per-test-coverage.py`

this file is not valid python, neither `black` or `python` can parse it. Since that's the case, can we change the extension on it so that we don't have to create a lot of exceptions in our CI and pre-commit hook?

https://github.com/llvm/llvm-project/pull/65242