Packages: Update Karma config for new structure

[swissspidy]

Feature Description

Our Karma configuration is currently a bit all over the place. Need to figure out how we can best improve this to make it more manageable.

• Dashboard and editor both have a fixture.js file and they actually have quite some code duplication there (ComponentStub, actPromise, HookExecutor, etc.)
• There's assets/src/karma-fixture which does a lot of different things:
  • Set up test-root for tests
  • Add virtual cursor
  • Add custom matchers inspired by jest, jest-dom, and axe
  • Add custom jasmine reporter to take snapshots
  • Implement pause/debug mode
  • Define describe.only & co (as if it were Jest), even though it's never actually used
  • Provide actual fixtures and component stub
• Then there are the two karma configs for each project, which are also very similar

Alternatives Considered

Additional Context

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Acceptance Criteria

Implementation Brief

[swissspidy]

Just discussed with @sayedtaqui that we can go ahead with just moving this to a new package for now, basically resurrecting #6276. If we realize this needs a different structure later on, refactoring should be easy.