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

[NAKAMURA Takumi via llvm-commits]

chapuni wrote:

I wonder this would suppress `check-llvm-unit` out of both `check-llvm` and `check-all`. Intentional?

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