Moving higher level cancel merge train spec to unit level

What does this MR do and why?

Current test case: https://gitlab.com/gitlab-org/gitlab/-/quality/test_cases/347665 is slow due to it being a higher level test case and using capybara.

Moving this spec to a lower level test to decrease runtime

Screenshots or screen recordings

N/A

How to set up and validate locally

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

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.

Related to gitlab-org/quality/quality-engineering/team-tasks#1704 (closed)