[QA] Extend create_merge_request_spec to set assignee to MR

What does this MR do?

• This MR extends https://gitlab.com/gitlab-org/gitlab-ce/blob/master/qa/qa/specs/features/browser_ui/3_create/merge_request/create_merge_request_spec.rb by setting an assignee to the merge request.
• From now on test will click 'Assign to me link' and will verify that assignee is correct.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/52704

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer