Instead of never finishing the response which works differently across
browsers, stall the required script.
This makes "DOMContentLoaded" to never fire and properly tests the
"commit" signal.
- Separate worker for some Android tests.
- Use png comparator for some screenshots tests instead of buffer
equality.
- Skip drag&drop tests on Android.
- Various timeout fixes.
Instead of requiring all frames in the subtree to receive a particular
event, we rely on the browser's definition of load and DOMContentLoaded.
This changes logic in a few edge cases:
- Some browsers do not emit load event upon window.stop() at all.
- DOMContentLoaded does not wait for subframes, so they might not be
ready when passing `{ waitUntil: 'domcontentloaded' }`.
`networkidle` preserves the old logic.
