Update LDAP admin sync test to add and remove a user's admin status

What does this MR do?

Updates LDAP admin sync test to also ensure that admin status of the user is removed from Gitlab when removed from LDAP.

Related to: gitlab-org/quality/testcases#105 (closed)

Depends on: gitlab-qa!279 (merged)

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