ktest: Allow a test to override REBOOT_ON_SUCCESS

The option REBOOT_ON_SUCCESS is global, and will have the machine reboot
the the box if all tests are successful. But a test may not want the
machine to reboot, and perhaps have the kernel it loaded be used to
install the next kernel. Or the last test may set up a kernel that the
user may want to look at. In this case, the user could have the global
option REBOOT_ON_SUCCESS be true, but if a test is defined to run at the
end, that test can override the global option and keep the kernel it
installed for the user to log in with.

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