||Formatter to produce results matching the Chromium JSON Test Results format.
Notably, each test has an "artifacts" field that is a dict consisting of
"log": a list of strings (one per subtest + one for harness status, see
_append_test_message for the format)
"screenshots": a list of strings in the format of "url: base64"
||Formatter that produces results in the format that wptreport expects.
||Formatter that outputs screenshots in the format expected by wpt.fyi.