This patch:
- updates documentation to lead users from `TestConfig.snapshotDir` and
`testInfo.snapshotSuffix` to `TestConfig.snapshotPathTemplate` as a
better and more flexible alternative.
- drops `{snapshotSuffix}` from documentation
- stops using `snapshotSuffix = ''` in our own tests and switches us
to the `snapshotPathTemplate`.
- adds `{testName}` token.
- 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.
