Changes

| Add structured logging to mochitest

| Traditionally at Mozilla, test harnesses have written test results as arbitrary strings to stdout. Other tools that want to gather data about test runs are forced to parse the entire logfile produced by capturing stdout, and using a set of regexes on each line in order to identify test results, test summaries, and other items of interest.

We'd like to replace this very inefficient approach to test logging with a more robust mechanism: "structured logging". In this approach, a test harness writes a file containing data in JSON format for each test event that needs to be logged, as well as producing human-readable data on stdout. Other tools can then easily parse the JSON data when they want to know what happened during the test run.