E2E tests for UI defined pipeline variable inheritance

Description of the test

Related to #213729 (closed) & #355749 (closed)

Feature flag roll out issue #355572 (closed)

Test cases:

• https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/358197 - child and downstream pipeline can inherit UI defined variable when pipeline_variables is true
• https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/358199 - child and downstream pipeline cannot inherit UI defined variable when pipeline_variables is false
• https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/358200 - child and downstream pipeline can inherit UI defined variable by default

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).
• Ensure that a created resource is removed after test execution. A Group resource can be shared between multiple tests. Do not remove it unless it has a unique path. Note that we have a cleanup job that periodically removes groups under gitlab-qa-sandbox-group.
• 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.