Set up frontend testing framework

With my limited knowledge, here's what I'm thinking:
- Unit tests for backend classes. I think phpspec is the preferred tool here – we already have one spec thanks to @franzliedke.
- Functional/integration tests for the JSON-API. (Not sure what tool would be best for this?)
- Functional/integration tests for the extension API. (Ditto?)
- Unit tests for front-end utils, components, helpers, and models. (Probably using QUnit?)
- Acceptance tests for the whole thing.

Thoughts?

Start by reviewing the existing phpspec example mentioned in the issue, then map the proposed coverage across backend classes, the JSON-API, the extension API, frontend utils, components, helpers, models, and whole-application acceptance tests. The issue does not name files, test entry points, or a decided tool, so the testing scope and definition of done need to be established before implementation.

Written by the indexing model from the issue text.