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

[Paul Kirth via llvm-commits]

================
@@ -0,0 +1,32 @@
+# Check the cat command
----------------
ilovepi wrote:

```suggestion
## Test the cat command
```
Check can sometimes be easy to miss w/ `CHECK` directives, and we often use `##` to indicate that a line is commentary, and not intended to be part of the lit/FileCheck directives.

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