msglist: Also test scroll position after ScrollToBottomButton was pressed

Author: sirpengi

test/widgets/message_list_test.dart

@@ -120,6 +120,7 @@ void main() {
       await tester.tap(find.byType(ScrollToBottomButton));
       await tester.pumpAndSettle();
       check(isButtonVisible(tester)).equals(false);
+      check(scrollController.position.pixels).equals(0);
     });
   });
 }