This commit adds a percent change to the duration on individual test
output lines. This is conditionally displayed based on the presence
of a testrepository times.dbm file and data in that file for the test
being run. If there is useable data subunit-trace will now use the
runtimes from there to display how the current run has changed from
the dbm file. A new threshold option is added to optionally specify a
minimum percent change to be used to determine whether to display the
value or not.
Change-Id: I3d68425f48114531a78cab08c353111648ce3911
bbc8b8f541