zdtm: check one-shot posix timers

With the patch zdtm will check not only periodic timers, but also
one-shot ones.

 Set one-shot timer to expire in a very long interval (INT_MAX in the
 test), and check its remaining value after checkpoint/restore. The
 following relationship must hold:

 initial_value - remaining_value = time_passed

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>