Add test for git http operations for SSO enforced SAML group

What does this MR do?

Adds a test to that exercises Git operations over HTTP for a SSO enforced SAML group.

Related issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/11803

Depends on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13485

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• [-] Style guides
• [-] Database guides
• Separation of EE specific content

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team