ktest: Ignore warnings during reboot

The reboot just wants to get to the next kernel. But if a warning (Call
Trace) appears, the monitor will report an error, and the reboot will
think something went wrong and power cycle the box, even though we
successfully made it to the next kernel.

Ignore warnings during the reboot until we get to the next kernel. It
will still timeout if we never get to the next kernel and then a power
cycle will happen. That's what we want it to do.

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