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

[Paul Kirth via llvm-commits]

================

----------------
ilovepi wrote:

diff returns an error code when files are different. You shouldn't need to use a check line at all, since the command itself should fail.

Here's an example from LLD's tests, but other places do somthing similar. https://github.com/llvm/llvm-project/blob/72a514f42722ad6f6e6baee8e2d75150f9369a66/lld/test/ELF/stdout.s#L10

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