Don't drop user dir deltas when server leaves room (#10982)

Fix a long-standing bug where a batch of user directory changes would be
silently dropped if the server left a room early in the batch.

* Pull out `wait_for_background_update` in tests

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

tests/unittest.py

@@ -317,6 +317,15 @@ class HomeserverTestCase(TestCase):
             self.reactor.advance(0.01)
             time.sleep(0.01)
 
+    def wait_for_background_updates(self) -> None:
+        """Block until all background database updates have completed."""
+        while not self.get_success(
+            self.store.db_pool.updates.has_completed_background_updates()
+        ):
+            self.get_success(
+                self.store.db_pool.updates.do_next_background_update(100), by=0.1
+            )
+
     def make_homeserver(self, reactor, clock):
         """
         Make and return a homeserver.