Fix flakiness in Group SAML specs and some improvements

What does this MR do?

1. Fixes "User already taken" issue by using a different SAML user for each test
2. Fixes failure due to unsuccessful logout
3. Moves removal of feature flags to the after block of the context where they were added
4. Removes the workaround put in place due to the bug https://gitlab.com/gitlab-org/gitlab/issues/35365 (now fixed)
5. Fixes an issue where screenshot captured is of the wrong screen due to UI steps in after block. Capybara captures screenshots in the after block. Any after block in the test would run first causing the screenshot to be captured after the test's after block is run.

Closes #37308 (closed)

Fixes 1st part of #37374 (closed)

Conformity

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