[llvm] [NFC][LLVM-Tests] Specialize test suite for LLVM unit tests (PR #161442)

[Rahul Joshi via llvm-commits]

jurahul wrote:

IIRC check-llvm and check-all are their own standalone targets independent of the check-llvm-* targets, but will check 

https://github.com/llvm/llvm-project/pull/161442