zdtm/mountpoints: wait when a child process opens descriptors
zdtm/mountpoints: wait when a child process opens descriptors

Otherwise we can see this error:
5: Old files lost: set([])
5: New files appeared: set(['5', '6'])

Reported-by: Mr Jenkins
Signed-off-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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