Consolidate labels sidebar rspec tests [RUN AS-IF-FOSS]

What does this MR do and why?

-Refactor rspec test for due date widget in issue board and issue board sidebars.

• Unquarantine and fix test from #27992 (closed) -Creates one shared example with more coverage for both pages.

Previous iterations of sidebar tests consolidation

• Milestone widget: !76513 (merged)
• Due date widget: !76718 (merged)

Fixes style and port error to new labels widget. To reproduce, attempt to create a label with a name that is already taken.

Screenshots or screen recordings

Before (only in create issue form)	After (everywhere new labels widget is used + new issue form)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #347496