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

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connieyzhu wrote:

It is not, but I believe it does test a special case where a file does not end with a newline. Do you think this is worthwhile to test, or should a newline be added?

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