Enable Usage Overview visualization for project namespaces

What does this MR do and why?

Enables the Value Stream Dashboard's Usage Overview visualization for project namespaces – previously, it was only available for group/subgroup namespaces. It also refactors the tests in ee/spec/features/projects/analytics/dashboards_spec.rb to help improve performance.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

1. Please make sure you have a GitLab Ultimate license.
2. Feature.enable(:project_analytics_dashboard_dynamic_vsd)
3. Create a new (or choose an existing) project, grab its parent group's ID and invoke the seed script: GROUP_ID=<group-id> FILTER=vsd_overview_counts SEED_VSD_COUNTS=1 bundle exec rake db:seed_fu
4. Navigate to the project above and, from the left sidebar, click the Analyze > Analytics dashboards link
5. The project dashboard list should appear, select Value Stream Dashboard
6. Verify that the Usage overview panel displays the project's data.

Related to #456893 (closed)