tests/pm_pc8: add "stay" mode

tests/pm_pc8: add "stay" mode

I used to have a binary that would just disable all the screens - so
we can enter PC8/runtime PM - and then sleep forever. I used this
binary many times while debugging PC8 and runtime PM, and I also sent
the binary to many people so they would be able to test these things
without X running.

Since pm_pc8 already implements everything that the separate binary
needs, and it even has some additional code to try to configure the
environment to actually reach PC8, it's easier to just ask people to
run "sudo ./pm_pc8 --stay" instead of sending them a file, asking them
to compile it, setup the environment, and then run it.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>