Export to DB results in local timezone for test_runs.started_at and test_runs.finished_at (and others)

[trwm]

Since output.xml is generated with local time, but without timezone information, dbbot could/should adjust all times to UTC.
For test_runs.imported_at this is already done by using:
'imported_at': datetime.utcnow()

However test_runs.started_at, test_runs.finished_at and other fields are stored without adjustment and therefore they are stored in local time. This leads to problems when further processing the data, since some information is stored in UTC and others is stored in local time.

Suggest to adjust _format_robot_timestamp(timestamp) (robot_results_parser.py) to e.g.:
return datetime.strptime(timestamp, '%Y%m%d %H:%M:%S.%f').astimezone(timezone.utc) if timestamp else None