Restore all mocked values after tests

In response to [1].

This prevents mocked values leaking between tests [2]. I did try to
set 'mockRestore: true' in jest.config, but that means the restore
happens before _every_ test, which isn't what we want.

[1]: https://github.com/alphagov/notifications-admin/pull/3878#discussion_r631060116
[2]: https://jestjs.io/docs/jest-object#jestrestoreallmocks

tests/javascripts/support/teardown.js

@@ -2,3 +2,5 @@ window.jQuery = null;
 $ = null;
 
 delete window.GOVUK;
+
+jest.restoreAllMocks();