Update ee/spec/features/groups/group_overview_spec.rb to reflect tests for the new Security Dashboard

Recently we have enabled the first_class_vulnerabilities feature flag in Staging. As part of #207373 (closed) - we are displaying the new Security Dashboards behind this feature flag. This requires tests to be updated in the Backend to reflect the changes.

Since !28652 (merged) is almost merged, and we're tight on time - we have intentionally left out updating the Backend Test for the Security Dashboard being loaded when the feature flag is enabled.

This issue is created to track the addition of those tests.