Enhance LDAP integration tests to cover 2FA

Creating an issue to track this test gap since this was raised as a priority1 in GitLab QA bug in production.

Related bug: https://gitlab.com/gitlab-org/gitlab-ce/issues/46661 Related MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19115

Pointers from Stan on how to go about this.

That should be an interesting exercise since you'd need to generate the OTP code (or just call User#current_otp from the Rails console).