Milestone tests

What does this MR do?

This MR is designed to fix gitlab-qa#231 (closed).

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This MR was necessary to expand the functionality of testing. Currently the test suite does not contain tests for attaching a milestone to a merge request

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

gitlab-qa#231 (closed)

/cc @rymai