Similar to r295116, add an additional 1 second sleep after calling ggatel

before calling dd to defeat a race when writing out to the geom_gate(4)
device

MFC after: 1 month
Reported by: Jenkins
Sponsored by: EMC / Isilon Storage Division

tests/sys/geom/class/gate/3_test.sh

@@ -27,6 +27,7 @@ if ! ggatel create -u $us /dev/$work; then
 	exit 1
 fi
 
+sleep 1
 dd if=/dev/${src} of=/dev/ggate${us} bs=1m count=1 conv=sync
 sleep 1