beforeEach function
Registers a function to be run before tests.
This function will be called before each test is run.
If this is called within a describe/context, it applies only to tests in that
describe/context. The body
will be run after any set-up callbacks in parent describe/context or
at the top level.
Each callback at the top level or in a given describe/context will be run in the order they were declared.
Implementation
void beforeEach(Future<void> Function(WidgetTester tester) body) {
_widgetDeclarer.setUp(body);
}