lib/drmtest: ducttape over fork race

lib/drmtest: ducttape over fork race

Whatever the reason (and I've thought there isn't one) if we fork and
kill right away the child seems to not reliably die. We can work
around this little race by forcing the default SIGQUIT handler. This
should break anything since we reset our atexit handling anyway, so if
the helper needs any atexit handling the special signal helpers will
be reinstated.

Note that inserting sufficient amounts of printf between the fork and
kill makes this unnecessary.

While add it also add the retry loop for the waitpid call, in case
there's another guy constantly interrupting us.

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