I have check this link:
https://github.com/flutter/flutter/blob/master/packages/flutter/test/material/bottom_navigation_bar_test.dart

It use tester.tap to drive one of BottomNavigationBarItem. but It is not very convenient if app support localization.
I think it add key can more flex to test.