[ooni-dev] test_start_time in JSON reports

[David Fifield]

The YAML reports had two time fields:
	start_time: timestamp of the start of ooni-probe run
	test_start_time: timestamp of the start of each individual test
Within a single report file, start_time was constant, while
test_start_time would advance with each successive test, depending on
how long each test took to run.

The JSON format reports have just one of the fields, test_start_time,
but it confusingly appears to have the same meaning as start_time in the
old YAML reports (it doesn't change within a report file):
	test_start_time: timestamp of the start of ooni-probe run

It might be because of this code:
https://github.com/TheTorProject/ooni-pipeline/blob/355ac1780f1f05eefb9ea3bf5b5c0148904e888c/pipeline/batch/daily_workflow.py#L521
        entry['test_start_time'] = datetime.fromtimestamp(entry.pop('start_time',
                                        0)).strftime("%Y-%m-%d %H:%M:%S")

Some tests can take many minutes or hours to run, so by the end, the
JSON test_start_time might be far off from the real time when it was
run. Is there a way we could get both timestamps in each record again?

What I'm doing now is incrementing a counter according to the
test_runtime field of each record, and adding that counter to the
test_start_time in order to estimate the individual test's start time.
But I feel that is only approximate, and some older reports do not have
test_runtime.