[llvm] [offload][cmake] Add option to include offload unittests in check-all (PR #211633)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 commented:

Is there a reason to not do this all the time? Most projects just run the unit tests with the lit tests, right?

https://github.com/llvm/llvm-project/pull/211633