E2E test for SAML SSO authentication for merge request password approval

Summary

Add an e2e test of SAML SSO authentication for merge request password approval.

Requirements

• SAML SSO orchestration

Steps

1. Enable SAML SSO and disable authentication via the standard sign-in tab.
2. Create a project with require user password to approve enabled.
3. Create an MR that requires approval.
4. As another user, approve the MR.
5. Verify that the user is required to re-authenticate via SSO.
6. Verify that the MR is approved and can be merged.

Links

• Docs: https://docs.gitlab.com/ee/user/project/merge_requests/approvals/settings.html#require-user-password-to-approve
• Epic: gitlab-org&11084