Extend Create New Merge Request test to also add a label

What does this MR do?

Extends the Create New Merge Request scenario test to provide a label and apply it to the created Merge Request.

Adds a Label resource factory and new Page Objects with basic support for creating new labels.

Closes gitlab-qa#233 (closed).

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

Why was this MR needed?

Additional coverage was requested for this test.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

gitlab-qa#233 (closed)