[Offload] Check plugins aren't already deinitialized when tearing down (#148642)
This is a hotfix for #148615 - it fixes the issue for me locally.
I think a broader issue is that in the test environment we're calling
olShutDown from a global destructor in the test binaries. We should do
something more controlled, either calling olInit/olShutDown in every
test, or move those to a GTest global environment. I didn't do that
originally because it looked like it needed changes to LLVM's GTest
wrapper.