E2E: Signup Page data-testid Migration

What does this MR do and why?

Relates to gitlab-org/quality/quality-engineering/team-tasks#1931 (closed) and gitlab-org/quality/quality-engineering/team-tasks#1735

In order to better unify our E2E testing framework with lower level tests and enable developers to more easily contribute to E2E tests, this MR migrates the data-qa-selectors to data-testids in the GitLab::Page::Main::SignUp and QA::Page::Registration::SignUp pages.

Please also note these conventions we will be following, from gitlab-org/quality/quality-engineering/team-tasks#1822 (closed):

• Use kebab-case for testid testid="kebab-case-attribute"
• Use strings for page elements - element :push_the_button becomes element 'push-the-button'

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

A green pipeline for e2e:package-and-test-ee and review-qa-* jobs should be sufficient to validate these changes ✅

You can also run qa/qa/specs/features/ee/browser_ui/11_fulfillment/purchase/user_registration_billing_spec.rb locally from the qa directory with:

QA_GITLAB_URL=<gdk url and port> GITLAB_INITIAL_ROOT_PASSWORD=<gdk root password> WEBDRIVER_HEADLESS=false bundle exec rspec qa/specs/features/ee/browser_ui/11_fulfillment/purchase/user_registration_billing_spec.rb

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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