Port merge request widget tests to VTU and improve tests

What does this MR do?

Tests of the merge widget will now use VTU and this solidify the tests and remove a bunch of false positive. A following MR will be done to remove the tests on computed properties and instead focus on the user experience.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Closes #235744 (closed)