This moves some expect() matchers tests from test runner tests to page tests,
because these are implemented through a library call anyway.
Makes tests more readbable, faster and easier to test specific details.
- Never use open shadow root for highlight. This messes up
our selectors that accidentally match internal preview elements.
- Remove failing electron test that we do not care about.
- Skip `channels.spec.ts` in non-default mode.
- Source now lives at `src/test`.
- Former folio tests live at `tests/playwright-test`.
- We use `src/test/internal.ts` that exposes base test without
Playwright fixtures for most tests (to avoid modifications for now).
- Test types live in `types/testFoo.d.ts`.
- Stable test runner is installed to `tests/config/test-runner` during `npm install`.
- All deps including test-only are now listed in `package.json`.
Non-test deps must also be listed in `build_package.js` to get included.
