chore(package.json): add cheerio dependency

It's tinier than jsdom, and seems to work okay for generating the fixtures.
chore(tests): implement e2e test harness outside of docs app

Included:

- A sample test fixture
- A sample test
- Server middleware to serve the E2E harness
- Convenient test helpers to simplify loading the right fixture

Closes #9557
Closes #9527