Add tenant migration unittests for op_observer_impl.

Tenant migrations has added specific requirements to OpObserverImpl.

It would be easier for future developers that have to edit OpObserverImpl to debug localized unittest failures in op_observer_impl_test.cpp first before having to dive into high level jstests.

The unittests would be particularly helpful because many op observer functions have very particular combinations of clauses in if-statements that are tricky to alter without changing existing behavior.