ktest: Add check for bug or panic during reboot

Usually the target is booted into a dependable kernel when a test
starts. The test will install the test kernel and reboot the box. But
there may be a time that the kernel is running an unreliable kernel and
the reboot may crash.

Have ktest detect crashes on a reboot and force a power-cycle instead.

This can usually happen if a test kernel was installed to run manual
tests, but the user forgot to reboot to the known good kernel.

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