Add specs for assigning an issue to an epic during the create

What does this MR do?

This MR started as adding support for assigning an epic to an issue (during create) but there has already another MR been merged during the review that adds that.

So this MR only adds additional specs.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Issue: #13847 (closed)