lib/drmtest: make signal process cleanup more robust

lib/drmtest: make signal process cleanup more robust

If we skip a test and and fail somewhere the parent might die before the
child. So add some cleanup to handle this case. Also make sure that the
parent indeed waits for the child to die.

This is required to make the latest version of the piglit runner happy,
it tends to wait for all zombies to disappear.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>