test(mobile): fix missing wait on finder for CongestionRatingRobot.countPrefixed()

Author: mrzzy

integration_test/e2e_routing_test.dart

@@ -55,10 +55,6 @@ void main() {
 
       // transition: home screen -> congestion rating screen
       final congestionRobot = CongestionRatingRobot(tester: tester);
-      // check at least one congestion point is listed
-      final nCongestionPoints =
-          await congestionRobot.countPrefixed(WidgetKeys.congestionPointPrefix);
-      expect(nCongestionPoints, greaterThan(0));
       // check congestion markers are drawn for every congestion point
       // check at least one route step is shown
       final nRouteSteps =

integration_test/robots/congestion_rating_robot.dart

@@ -25,7 +25,7 @@ class CongestionRatingRobot {
   }
 
   Future<int> countPrefixed(String prefix) async {
-    final cards = find.byKeyPrefix(prefix);
+    final cards = await find.byKeyPrefix(prefix).wait(tester);
     return cards.evaluate().length;
   }