Sometimes, we are unable to take a frame snapshot. The most common
example would be "frame is stuck during the navigation in Chromium",
where we cannot evaluate until the frame is done navigating.
In this case, use all other frames and just stub the failing ones
with "Snapshot is not available". Chances are, noone will even see
this frame because it's an invisible tracking iframe.
This introduces basic tracing enabled in our tests.
What is captured:
- network resources;
- snapshots at the start of most actions;
- snapshot after the test failure.
How this integrates with test runner:
- context fixture calls private method context._initSnapshotter() and uses Tracer to trace all events;
- all tests share a single test-results/trace-storage directory to store blobs;
- each test has its own trace file.
- npm run show-trace opens a bare-minimum trace viewer that renders snapshots.
