Add E2E test for prefill variables

Description of the test

As a corrective action of the incident gitlab-com/gl-infra/production#7618 (closed), we decided to have an E2E test for prefill variables.

• The cause MR: !94778 (merged)
• The revert MR: !95682 (merged)

Test case https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/371204 :

1. Add a CI YAML config with variables:
   • TEST1 prefill variable (with value and description)
   • TEST2 variable with just description, no value
   • TEST3 with only value
   • TEST4 in regular format
2. Open the Run Pipeline page.
3. Check that:
   • TEST1 variable is present with correct prefill description and value
   • TEST2 variable is present with correct description
   • TEST3 and TEST4 variables are not shown.

Checklist

• Confirm the test has a testcase: tag linking to an existing test case in the test case project.
• Note if the test is intended to run in specific scenarios. If a scenario is new, add a link to the MR that adds the new scenario.
• Follow the end-to-end tests style guide and best practices.
• Use the appropriate RSpec metadata tag(s).
• Most resources will be cleaned up via the general cleanup task. Check that is successful, or ensure resources are cleaned up in the test:
  • New resources have api_get_path and api_delete_path implemented if possible.
  • If any resource cannot be deleted in the general delete task, make sure it is ignored.
  • If any resource cannot be deleted in the general delete task, remove it in the test (e.g., in an after block).
• Ensure that no transient bugs are hidden accidentally due to the usage of waits and reloads.
• Verify the tags to ensure it runs on the desired test environments.
• If this MR has a dependency on another MR, such as a GitLab QA MR, specify the order in which the MRs should be merged.
• (If applicable) Create a follow-up issue to document the special setup necessary to run the test: ISSUE_LINK
• If the test requires an admin's personal access token, ensure that the test passes on your local environment with and without the GITLAB_QA_ADMIN_ACCESS_TOKEN provided.