Currently, the xml report only reports the total test execution time, and do not provide data on the time
taken for individual test cases. This change add a _duration attribute to the base test case that is then
populated during tearDown and added to a 'test_time' attribute on the result object. The duration is
determined using a the same mechanism as the test_metrics reporting. The time is reported in seconds.
Change-Id: I41931e12e13e073debd37403fcc0030c26041fb0
96f747133f