JSON file output flag

[riglar]

The ability to pass:

• --json-file test_artifacts/myoutput.json
• --json-file (which then defaults to <upload_id>.json)

This will capture the json output (usually send to stdout via --json) and write it to a file.

This lets CI users see stdout progress whilst still providing a route for machine-readable results parsing.

[riglar]

In 3.6.4