When a subunit stream for a testcase doesn't contain start end
enddate, the duration can't be calculated which leads to a:
ValueError: could not convert string to float
Check now if the duration is an empty string and add basic test
coverage based on ddt for the subunit_trace command.
Change-Id: I9953019794ba53fcfcb20e32fecbe94da22c9565
9832648353