zdtm: improve checking file descriptors and memory regions (v2)

Now we check them for all test processes

v2: Busybox doesn't have truncate
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>