Currently,  the failpoint PrimaryOnlyServiceSkipRebuildingInstances added in SERVER-51052 can’t be used in tests that use primary only services. It forces 
PrimaryOnlyService::_rebuildInstances to return early without setting _state to kRunning and do _rebuildCV.notify_all(). This causes PrimaryOnlyService::getOrCreateInstance to hang since it waits for _rebuildCV when _state is kRebuilding.

Instead of making _rebuildInstances return early, the failpoint should just skip this code block.