Fix global state in registrations_controller_spec
What does this MR do?
This spec would mess with global state used for captchas, but not reset the state in all cases. This would then lead to the spec spec/controllers/snippets_controller_spec.rb failing, but only on a fork of gitlab-ee. To fix this we ensure the state is properly reset after running the specs that change this state.
This is one of several steps needed to get the tests for https://gitlab.com/gitlab-org/gitlab-foss to pass.