Draft: Splits usage quotas specs into three specs for each tab

What does this MR do and why?

This MR reorganises Usage Quotas specs into four files: one is general specs, other three match three tabs that we have on Usage Quotas page.

Previously we had two spec files: ee/spec/features/groups/seat_usage/seat_usage_spec.rb and ee/spec/features/groups/usage_quotas_spec.rb, containing mixed specs for all three tabs. It doesn't add or remove specs, just splits them.

Screenshots or screen recordings

n/a

How to set up and validate locally

rspec ee/spec/features/groups/usage_quotas/

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.