Group SAML SSO Specs: Use a new user for the tests instead of root

What does this MR do?

#254376 (comment 421122881) originally failed because of 404 issues that should be fixe by now. However, the failure revealed another problem. The test is not idempotent and the second try of the test fails because the user's link to the idp has already been made. This MR fixes that be create and using a new user for each try and then deletes that user at the end.

Closes #254376 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• 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