[llvm] [llvm-lit][test] Precommit tests for lit's built-in cat command (PR #101530)

[Paul Kirth via llvm-commits]

https://github.com/ilovepi commented:

This is basically LTTM, but I wonder if we need more tests for `cat`? Maybe one that diffs the original file to a temp file?

Maybe something like:
```
RUN: cat %s/txt-files/allchars > %t
RUN: diff %s/txt-files/allchars %t
```

https://github.com/llvm/llvm-project/pull/101530