lib/drmtest: Avoid calling exit handlers multiple times

lib/drmtest: Avoid calling exit handlers multiple times

- reset the count when forking
- don't add the same handler multiple times
- don't restore the exit signal handlers in the forked helper
  process
- reset the exit handler count once called to make sure we don't call
  it multiple times when dying
- don't wait for the signal helper if it's gone already

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