[llvm] [llvm-lit][test] Precommit tests for curly braces in lit internal shell (PR #102976)

[Paul Kirth via llvm-commits]

ilovepi wrote:

There's some value in showing that each test used to fail, but now passes. I'm not sure it needs to be particularly exhaustive, but whatever basic tests you're considering are probably reasonable to include in this patch. Typically its nice if none of the RUN lines change, but you can easily spot a difference in the test behavior. I know I appreciate that in the middle or backend tests I review.

https://github.com/llvm/llvm-project/pull/102976