[llvm] [llvm][Docs] Make it clear where lit test files live (PR #124121)

[Mats Jun Larsen via llvm-commits]

https://github.com/junlarsen approved this pull request.

LGTM. There was also some confusion on regression vs unittest, but I think the docs explain that well enough already.

https://github.com/llvm/llvm-project/pull/124121