zdtm: mount tmpfs into /run in a test root

iptables creates /run/xtables.lock file and
we want to have it per-test.

(00.332159)      1: 	Running iptables-restore -w for iptables-restore -w
Fatal: can't open lock file /run/xtables.lock: Permission denied
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/lib/ns.c

@@ -98,6 +98,11 @@ static int prepare_mntns(void)
 		return -1;
 	}
 
+	if (mount("zdtm_run", "/run", "tmpfs", 0, NULL)) {
+		fprintf(stderr, "Unable to mount /run: %m\n");
+		return -1;
+	}
+
 	if (umount2("./old", MNT_DETACH)) {
 		fprintf(stderr, "umount(./old) failed: %m\n");
 		return -1;