[llvm] [Offload] Stop using global ctor/dtors in Offload unit tests (PR #149299)

[Callum Fare via llvm-commits]

callumfare wrote:

We do have reference counting now. This might not be strictly necessary then, I'll have a closer look at the original problem and see what's actually happening

https://github.com/llvm/llvm-project/pull/149299