Add e2e tests for CI minutes purchase

What does this MR do and why?

• Add purchase CI tests with an active subscription
• Add purchase CI tests with existing CI minutes packs
• Add necessary elements on page

Screenshots or screen recordings

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How to set up and validate locally

Run

bundle exec bin/qa Test::Instance::All https://staging.gitlab.com -- qa/specs/features/ee/browser_ui/fulfillment/purchase/purchase_ci_spec.rb

MR acceptance checklist

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