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

[James Henderson via llvm-commits]

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+## Tests to check cat -e output
+
+# RUN: cat -e %S/txt-files/allchars | FileCheck %s --check-prefix=ALLCHARS
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jh7370 wrote:

I think it would be worth in all of these test cases adding `--implicit-check-not={{.}}` to the FileCheck commands. This will prevent extra output between all the different checks from going unnoticed. You probably should also add `--match-full-lines` and `--strict-whitespace` to prevent whitespace being canonicalised by FileCheck. If you do this, you'll need to remove all whitespace from your check lines (between the colon and the actual thing being checked), and make sure hard tabs/spaces only appear where you'd expect them to.

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