ktest: Do not opencode reboot in grub setting

When setting the next kernel to boot to with grub, do not opencode
the reboot operation.  The normal reboot operation can be modified by
config options (namely POWERCYCLE_AFTER_REBOOT). This needs to affect
all reboots. Remove the opencoded reboot to make sure that any changes
to the reboot code also affect all reboots.

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