Fix spec violations in spec/frontend/notes/components/diff_discussion_header_spec.js

This spec file has some technical debt and/or antipatterns that need to be fixed before we can update our codebase to Vue3. This file may contain potential violations such as:

• spying on methods
• .setData() usage
• overriding method behaviour
• wrapper.vm or childComponent.vm modification
• Vue.extend

Please see the related epic for more details on how these violations can be addressed.