zdtm: Write .inprogress pidfile

This one will contain pid of the task living in a sub-ns and
waiting for the rest of the test to get daemonized.

Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Travised-by: https://travis-ci.org/xemul/criu/builds/170726663