[llvm] [Offload] Run unit tests as a part of check-offload (PR #156675)

[Michał Górny via llvm-commits]

mgorny wrote:

I think both issues I have been facing are fixed now: we have switched to building via `runtimes` and the test failures were fixed, so no more opposition to this change from me. That said, I think that tests on plain standalone builds are still broken.

https://github.com/llvm/llvm-project/pull/156675