Registers a function to be run after tests.
This function will be called after 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 before any tear-down 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 reverse of the order they were declared.
See also addTearDown, which adds tear-downs to a running test.
void tearDown(callback()) => _declarer.tearDown(callback);