Make release specs ready for vue3 migration

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR fixes first two problems described here.

The first one is removing Vue.extend() call from the spec file, which was very straightforward to implement.

The second is a bit trickier. After some digging I've realised the that the specs violating the convention were indirectly testing the behavior defined in one of the mixins that the original component under test was using. The best way would've been to refactor the whole thing, but components in question might be discarded entirely in some near future, so I decided to refactor the tests instead. I've created the new spec file to explicitly test the mixin, that was implicitly tested in the previous implementation. There I've provided a simplistic UI to trigger the mixin methods passing configurable parameters. This way the need of overriding the behavior of any methods was eliminated, and the test coverage should remain the same.

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