ktest: Fix bug where the test would not end after failure

The config STOP_AFTER_FAILURE is the number of seconds to continue
the test when a failure is detected. This lets the monitor record
more data to the logs and console that may be helpful in solving
the bug that was found.

But the test had a bug. If the failure caused multiple
"Call Trace" stack dumps, the start time to compare the
STOP_AFTER_FAILURE would constantly be reset. Only update the start
time at the first "Call Trace" instance.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>