[llvm] [NFC][ADT] Introduce a test harness for StringRefTest (PR #105500)

[Jakub Kuderski via llvm-commits]

kuhar wrote:

To me this seems like overkill for what we need to check that rbegin/rend point to the right place... For example, you can use `llvm::equal` to check that ranges match.

https://github.com/llvm/llvm-project/pull/105500