Add e2e tests from SaaS trial from billing page

What does this MR do and why?

• Add e2e test for starting trial from group billing page
• Updating existing test setup to use top-level group

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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/free_trial_spec.rb

MR acceptance checklist

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