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

[James Henderson via llvm-commits]

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@@ -0,0 +1,16 @@
+## Tests to check cat output
+
+# RUN: cat %S/txt-files/newline | FileCheck %s --check-prefix=NEWLINE
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jh7370 wrote:

Can we simply use `diff` here instead of FileCheck, like you do below?

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