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

[Michał Górny via llvm-commits]

mgorny wrote:

This change broke standalone builds. I think it is because `check-offload-unit` was broken in the first place, but now we can't run tests at all:

```
[57/59] cd /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/test && /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/temp/python3.13/bin/python3 /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/llvm-lit -vv -j 96 /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/test/unit
/usr/bin/python3.13: can't open file '/var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/llvm-lit': [Errno 2] No such file or directory
FAILED: [code=2] test/CMakeFiles/check-offload-unit /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/test/CMakeFiles/check-offload-unit 
cd /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/test && /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/temp/python3.13/bin/python3 /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/llvm-lit -vv -j 96 /var/tmp/portage/llvm-runtimes/offload-22.0.0.9999/work/offload_build/test/unit
ninja: build stopped: subcommand failed.
```

Other tests are using `add_offload_testsuite()` that seems to configure lit correctly. This one seems to assume that `llvm-lit` is configured somehow, but offload doesn't do anything like that.

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