[QA] Test push limit with admin user

What does this MR do?

Uses :requires_admin metadata to specify that a test needs an admin user.

Sets the push file size limit test to require an admin user.

Requires: gitlab-qa!248 (merged)

There is also an MR to enabled testing admin features on staging: gitlab-org/quality/staging!27 (merged) (and I've already added GITLAB_QA_ADMIN_ACCESS_TOKEN to the CI variables).

Closes gitlab-org/quality/staging#37 (closed)

Does this MR meet the acceptance criteria?

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