Fix Vue3 spec violations in app_spec.js

What does this MR do and why?

Fix Vue3 spec violations in spec/frontend/issues/show/components/app_spec.js. Note: I removed some specs in jest that were really just manually calling methods and asserting some arbitrary behaviours and moved them to a feature spec. Although a bit more slow to run, they represent the behaviour much better by actually testing that the alert that prevents navigating away is shown when it should.

Screenshots or screen recordings

none

How to set up and validate locally

Just a green pipeline and better tests 🌟

MR acceptance checklist

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Related to #391183 (closed)