In SERVER-135564 we added this `waitForCondition()` helper to make it more obvious where a hang might be coming from in a unit test.

It currently resides in results_merger_test_fixture.h, but it is a good util for broad use, so I propose moving it to src/mongo/db/query/util/ or some similar location.

Also worth doing a scout around the codebase to see if there are many other unit tests which could benefit from this.