[llvm] [LIT] Print discovered tests and percentages (PR #66057)

[via llvm-commits]

muiez wrote:

Hi, I think the following tests need to be updated as well to avoid breakages:
```
ignore-fail.py
shtest-define.py
shtest-env.py
shtest-not.py
shtest-pushd-popd.py
```

https://github.com/llvm/llvm-project/pull/66057