Add e2e tests for purchase storage with new route

What does this MR do and why?

• Add an e2e test for purchasing storage using new checkout flow (enabling :new_route_storage_purchase in tests)
• Update CI tests to use Helper::Plan module

Describe in detail what your merge request does and why.

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_storage_spec.rb

MR acceptance checklist

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