Increase test coverage

[bmos]

Test coverage was 47% when I was added as a maintainer. I've gotten it up to 67% but having 90+% should make maintenance easier and catch bugs before they go out to users.

[lordjabez]

Done in #221 and the follow-ups #222 and #223. The suite now runs fully offline against a local test server that doubles as an HTTP proxy. It covers element states and timeouts, clicks, cookies in both directions, remote drivers, proxy and User-Agent launch flags, and all HTTP verbs. Coverage went from about 81% to 99% across Chrome and Firefox, and the full CI matrix is green.