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

[Thomas Preud'homme via llvm-commits]

RoboTux wrote:

> I'm not trying to rush reviewers, but I want it to be clear that this patch is still in need of review due to recent changes. See [#65242 (comment)](https://github.com/llvm/llvm-project/pull/65242#issuecomment-1712676799) for a summary. I'm afraid the recent discussion of black might have buried that comment. Maybe I'm just not used to LLVM PR reviews yet.

I saw it but was waiting on the updated version fixing the python issue before giving it another look. I shall look at it today.

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