Add e2e tests for upgrade group subscription with existing add-on purchase

What does this MR do and why?

• Add e2e tests for upgrade group subscription with existing add-on purchase
• Update e2e tests to use Flow::Purchase module

Describe in detail what your merge request does and why.

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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/upgrade_group_spec.rb

MR acceptance checklist

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