Add e2e test of merge when pipeline succeeds

Description of the test

A smoke test of the Merge When Pipeline Succeeds feature

gitlab-org/quality/testcases#1684 (closed)

Check-list

• 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.
• 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