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

[Joel E. Denny via llvm-commits]

jdenny-ornl 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 <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.

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