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.
Source
Future<Null> runTest(Future<Null> testBody(), VoidCallback invariantTester);