Users of this unittest-fixture and all of its descendents should ensure as part of teardown/destruction all threads it spawned are joined, in particular the PinnedConnectionTaskExecutor threads. If these threads out-live the execution of the unittest, through PinnedConnectionTaskExecutorTest they may try and access the fixture, which will be destroyed after the unittest finishes.

The PinnedConnectionTaskExecutor tests themselves do this. But the mongot_cursor_test and task_executor_cursor_tests need to either have the fixture handle this for them.