Simplify Group SAML SSO specs and run SAML IDP as a service at runtime

What does this MR do?

Simplifies and refactors Group SAML SSO specs, splits them in multiple files and run SAML IDP service at runtime. This allows us to use different SAML enabled groups for tests thus isolating the tests from each other.

Also adds a file for common SAML flows.

One spec is quarantined due to a bug.

Gitlab-QA MR: gitlab-qa!379 (merged)

Closes #55242 (closed)

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content