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 <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>