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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> It appears that this breaks the ability to do `LIT_FILTER=offload/some-test ninja check-offload` with a test coming from the lit tests.
> 
> Leaving this here as an observation and something we likely want to fix at some point.

Hm, maybe the other targets do something slightly different to register their unit tests. Right now we just do `add_test`.

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