Add support for test_start_time and test_runtime keys in report.
testDone is no longer called so we can remove it from unittests.
Change the payload of the filename prefix files.

Setting a time too close to epoch makes UTC time conversions not work.
Fix the setting of the test runtime for tests with post processor
Fix time conversions to be all be in UTC
Merge pull request #356 from TheTorProject/feature/start_time_keys

Add support for test_start_time and test_runtime keys in report.