ktest: Add another monitor flush before installing kernel

On some tests that do multiple boots (patchcheck, bisect, etc), the build
of the next kernel to run may finish before the stable kernel has finished
booting. Then the install of the new kernel will fail when it tries to connect
as the machine has not finished the boot process.

Do one more monitor flush to make sure the machine is up and running before
trying to connect to it again.

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