Contributes to https://github.com/flutter/flutter/issues/130459
It adds test for
- `examples/api/lib/material/scaffold/scaffold.0.dart`
- `examples/api/lib/material/scaffold/scaffold.2.dart`
It also modifies the `scaffold.1_test.dart` because the only difference with scaffold 0 is the background color, so I figured the test should include it
