Adds a new structured-log formatter that produces a compact JSONL summary of a test run. It drops noise (non-ERROR log lines, process_output, mozleak_total), buffers test_start/test_end pairs into a single test_result record carrying start/end timestamps, and strips high-volume per-record fields (thread, pid, source, extra, tests, stackwalk_*). Differential Revision: https://phabricator.services.mozilla.com/D307017