Stop wrapping/prepending error messages so that we do not loose the stack trace. For this, update a few manually thrown errors with better messages (usually including a file path).
Speed up locations by doing manual `sourceMapSupport.wrapCallSite()` for a single call site. Performance gain in the runner process with 100 files x 100 tests each:
- 25% on the fresh run without babel cache;
- 80% on the cached run where babel is almost instant.
Also some obvious cleanups around stack traces (removing unused code).
- 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.
