Fix push mirror check script

What does this MR do?

Fixes this error in the push mirror status check script. This failure happened after adding specs and changing a line to require 'gitlab'. This changes it back and ensures the specs still work by adding require 'rspec/mocks'.

Test of this MR in the scheduled pipeline: https://gitlab.com/gitlab-org/quality/staging/-/jobs/5647131028

Check-list

• Ensure commits include Changelog: trailer
• Verify the change in all affected pipelines.