Registers a function to be run before tests.
This function will be called before each test is run.
callback may be
asynchronous; if so, it must return a Future.
If this is called within a test group, it applies only to tests in that
callback will be run after any set-up callbacks in parent groups or
at the top level.
Each callback at the top level or in a given group will be run in the order they were declared.
void setUp(callback()) => _declarer.setUp(callback);