Use MustSyncUntil to avoid a race

tests/federation_room_join_partial_state_test.go

@@ -1114,11 +1114,11 @@ func TestPartialStateJoin(t *testing.T) {
 		psjResult.FinishStateRequest()
 
 		// the /sync request should now complete, with the new room
-		response, _ = alice.MustSync(t, client.SyncReq{Since: nextBatch})
-
-		if !response.Get(syncJoinedRoomPath).Exists() {
-			t.Fatal("Sync should now include the joined room since resync is over")
-		}
+		nextBatch = alice.MustSyncUntil(
+			t,
+			client.SyncReq{Since: nextBatch},
+			client.SyncJoinedTo(alice.UserID, serverRoom.RoomID),
+		)
 	})
 
 	// test that a partial-state join can fall back to other homeservers when re-syncing