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

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

> But feel free to either update the docs to reflect the reality or post to discourse suggesting a policy change.

I'll plan to raise it in the [original RFC](https://discourse.llvm.org/t/rfc-document-and-standardize-python-code-style/68257) before proposing any changes.

Thanks for all your work in this area.  It is much appreciated.

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