Use after_teardown instead of teardown.

Users usually override the teardown method in their applications. In
some cases they don't call super on their implementation so the stubs
will not be cleaned.

after_teardown as the minitest documentation[1] stands is not to be used by
end users so we can use it.

[1] https://github.com/seattlerb/minitest/blob/34760e3b268bc1bb4ac5fe1a44ef1ff0a2f9bd4d/lib/minitest/test.rb#L192-197