Registers a function to be run once after all tests.
If this is called within a test group,
callback will run after all tests
in that group. It will be run before any tearDownAll callbacks in parent
groups or at the top level. It won't be run if none of the tests in the
group are run.
Note: This function makes it very easy to accidentally introduce hidden dependencies between tests that should be isolated. In general, you should prefer tearDown, and only use tearDownAll if the callback is prohibitively slow.
void tearDownAll(callback()) => _declarer.tearDownAll(callback);