Refactor spec file for split button

What does this MR do and why?

This refactors the spec file in preparation for this MR > !136716 (merged)

• Group all the "render" tests together for better organization and navigation
• Simplify the createComponent by passing a default button prop.
• Fix a vue warning error.

Screenshots or screen recordings

There are no UI/UX changes.

How to set up and validate locally

1. Run tests and it should pass without any errors

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.