ktest: Fix reboot on success stopping all reboots

The change to let individual tests decide to reboot the machine on
success of the entire test also prevented errors from rebooting
when an error was detected.

The "no_reboot" variable was only cleared if the test had
reboot_on_success set. But the no_reboot variable also prevents the test
rebooting when an error was detected even when REBOOT_ON_ERROR was set.

Add a new "reboot_success" variable that is used to determine if the
test should reboot on success and not touch the no_reboot variable.

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