Consolidate due date sidebar rspec tests [RUN AS-IF-FOLL

What does this MR do and why?

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

Creates one shared example with more coverage for both pages.

Previous iterations of sidebar tests consolidation

• Milestone widget: !76513 (merged)

Screenshots or screen recordings

No visual changes as it's a test refactor.

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #347496