Move Danger to test stage

What does this MR do?

This moves the Danger review to the test stage. The benefit is that it gives a few minutes for the developer to create the MR, having more chance that Danger can successfully find the MR and post a comment on it.

This also improves a bit danger/changelog/Dangerfile and danger/specs/Dangerfile (see specific commits).

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Conform to the code review guidelines
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conform to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• [-] End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49185.