This provides a machine-consumable way to get unit test results as they occur. With the json-realtime option, each test result is outputted on it's own line as "ugly" JSON, prefixed with a ##arc-unit## string.
Mostly this is to allow Harbormaster to parse test results as they are run, rather than parsing all the results at the end.
The ##arc-unit## string is a bit arbitrary, but I'm 99% sure we need a way to distinguish between random build output and Arcanist unit results (especially if there are other tools outputting blobs of JSON).