A Set of Tests for Checking that the ResponsiveSafeArea is Working as Expected

[psygo]

Some notes:

• I'm using a MaterialApp wrapper because the WidgetTester often complains if it isn't testing a Material widget, I don't know why.
• The first test tests that the contraints inside the LayoutBuilder don't take into account the artificial padding on top (artificial Status Bar). The second one shows that the ResponsiveSafeArea does recognize that difference.
• I think it would be nicer to have the ResponsiveSafeArea's contraints to be of the same type as the LayoutBuilder's, i.e., BoxConstraints instead of Size, for standardization.