Migrate billing_seats_usage_data_spec to billing_plans_spec

What does this MR do and why?

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/466077

In order to help prepare for https://gitlab.com/groups/gitlab-org/-/epics/9569+, we need to evaluate what E2E tests in GitLab that interact with purchase flows should be removed, migrated to CustomersDot E2E tests or moved to a lower level test.

This MR moves the assertions around seat usage counts from the E2E test ee/browser_ui/11_fulfillment/utilization/billing_seats_usage_data_spec.rb to the refresh seats context that already exists in the feature spec ee/spec/features/billings/billing_plans_spec.rb. The E2E test did not assert on any behavior within the purchase flows directly, and only verified the values that display for the subscription on the group billing page. Therefore, a mocked subscription should suffice in order to test this functionality.

Since the E2E test also asserted on the subscription start and end date, ee/spec/features/groups/billing_spec.rb was also updated to include an assertion for the end date since it already checks for the start date.

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.

How to set up and validate locally

1. Checkout the branch from this MR to your local GDK
2. In the gitlab directory, run the following commands:

bundle exec rspec ee/spec/features/billings/billing_plans_spec.rb

bundle exec rspec ee/spec/features/groups/billing_spec.rb

Checklist After Merging

• Archive test case https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/373506