Feature flag for group test coverage

What does this MR do?

Related to #215135 (closed)

Created a feature flag to hide the group test coverage table while we develop the feature. The backend and the frontend will be done separately, and this allows us to work in parallel.

No changelog needed since this is all behind a feature flag.

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