Fix mock.patch tearDown in test helpers
Fix mock.patch tearDown in test helpers

mock.patch.start() returns the new mock.Mock() instance by default.
Calling .stop() on this mock instance doesn't raise an exception
(since it's a mock), but doesn't remove the patch either.

mtekel/digitalmarketplace-admin-frontend,mtekel/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend,mtekel/digitalmarketplace-admin-frontend,mtekel/digitalmarketplace-admin-frontend,alphagov/digitalmarketplace-admin-frontend