Replace deprecated `.is`

What does this MR do?

.is will be deprecated in @vue/test-utils 1.x. Authors of VTU suggest replacing it with matching against tag name, but .is has another important piece of functionality - checking against component class.

This MR introduces new custom matcher .toBeVueInstanceOf and replaces remaining .is invocations with this custom matcher as a part of preparing of our upgrade (see &4255 (closed) and &3856 (closed) for details)

Screenshots

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