ktest: Add option REBOOT_SUCCESS_LINE to stop waiting after a reboot

When ktest.pl reboots, it will usuall wait SLEEP_TIME seconds of idle
console before starting the next test. By setting the
REBOOT_SUCCESS_LINE, ktest will not wait SLEEP_TIME when it detects the
line while rebooting to a new kernel.

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