Migrate CE MrWidgetOptions spec to @vue/test-utils

What does this MR do?

This is a mostly-straight-forward and nearly minimal migration; there are a lot of issues with the existing suite, e.g., mutating props directly; making assertions against component internals; inconsistent use of async/done/promise chains. Fixing these problems is well outside the scope of this, and can be addressed later.

Partly addresses #220476.

Screenshots (strongly suggested)

n/a

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done