zdtm.py: Test the --restore-sibling option
zdtm.py: Test the --restore-sibling option

The main difficulty here is that restored test becomes zdtm.py's kid
and after killing one we should wait() the test's zombie. W/o the
--restore-sibling test gets restored under existing init and gets
wait()-ed automagically.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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