runTest method
Call the testBody inside a FakeAsync scope on which pump can advance time.
Returns a future which completes when the test has run.
Called by the testWidgets and benchmarkWidgets functions to run a test.
The invariantTester argument is called after the testBody's Future
completes. If it throws, then the test is marked as failed.
The description is used by the LiveTestWidgetsFlutterBinding to
show a label on the screen during the test. The description comes from
the value passed to testWidgets. It must not be null.
Implementation
Future<void> runTest(Future<void> testBody(), VoidCallback invariantTester, { String description = '' });