zdtm: wait a test process instead of output file

zdtm.sh reports error immediatly, if a test process exited or crashed.

We should be fear, that pid can be reused. There are many place, which
will fail in this case. And a chance to get this situation is really small.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>