[QA] add tests for creating a branch using the UI

What does this MR do?

Add some test for the 'user creates a branch scenario' using GitLab's UI.

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

Why was this MR needed?

We are adding tests to gitlab-qa

Screenshots (if relevant)

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

What are the relevant issue numbers?

This is the basis of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15626 I extracted it so we can get this in faster as it may be of use to other tests currently being implemented.

/cc @grzesiek @felipe_artur