Use feature flags mixin in grouped security reports app

What does this MR do?

This follows up on @kushalpandya's comment here: !20381 (comment 255004769)

This is a small refactor of the grouped_security_reports_app.vue component to use the glFeatureFlagsMixin mixin instead of checking the global gon object. This change implies a refactor in the specs because the mountComponent() helper doesn't support the provide option which is required to properly stub the feature flag with the mixin, thus the helper had to be dropped in favor of Vue Test Utils' mount() function.

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