Refactor label specs for issue, MRs, and epics
What does this MR do and why?
Currently epics and merge requests don't test the addition and removal of labels. Move the specs from issues into shared examples.
This is in support for https://gitlab.com/gitlab-org/gitlab/-/issues/408676+
MR acceptance checklist
This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.
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I have evaluated the MR acceptance checklist for this MR.