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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

I'm going to assume that this causes some `ifdef` to be false which never registers the test or something.
```
-DLIBOMPTARGET_PLUGINS_TO_BUILD=host;cuda
```

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