The JournalFlusher initiated by ServiceContextMongoDTest uses the TickSource set on the ServiceContext. NamespaceMetadataChangeNotificationsTest sets a new TickSource on the ServiceContext, which deletes the original TickSource without any concurrency control. Therefore, we must adjust the ServiceContext's TickSource before the JournalFlusher is started up.

We could push NamespaceMetadataChangeNotificationsTest's ServiceContext::setTickSource() call into the ServiceContextMongoDTest base class before the JournalFlusher is started via StorageControl::startStorageControls. However, there are other unit tests, not currently failing, which also call ServiceContext::setTickSource(). Maybe we need some lower level test fixture for sharding setting TickSource the like??