Hide child epic icon on Roadmap for Premium accounts

What does this MR do?

Premiums account users don't have child epics, as a bug, we're displaying a child epics icon on the Roadmap for all users.

This checks the group features for sub epics support, if available the allowSubEpics flag is propagated with the store and used in the epic details component to hide/show the child epics icon.

This also adds the store to parent components specs that use @vue/test-utils/mount.

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Related to #216078 (closed)