Add purchase storage e2e tests

What does this MR do and why?

• Add e2e tests for purchase storage with active subscription and CI minutes pack
• Add a purchase flow module
• Update existing tests to use purchase flow

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/11_fulfillment/purchase/purchase_storage_spec.rb

MR acceptance checklist

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