out-of-space: Use busybox dd

Since rust-coreutils does not not provide dd yet, use busybox dd.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

run-test-out-of-space.sh

@@ -30,7 +30,7 @@ phase_test_update_does_not_start_if_disk_too_low()
 	local DISKSPACE=$(df -P -B 1M / | grep / | awk '{ print $2-$3-'$MAGIC' }')
 
 	# Fill the disk
-	dd if=/dev/zero of=/var/bigfile bs=1M count=$DISKSPACE
+	busybox dd if=/dev/zero of=/var/bigfile bs=1M count=$DISKSPACE
 
 	# Ensure the update did not apply
 	if apply_update_sync -d $DELTAFILE ; then
@@ -53,7 +53,7 @@ phase_update_stops_safely_if_disk_too_low()
 	# Fill the disk
 	local MAGIC=25
 	local DISKSPACE=$(df -P -B 1M / | grep / | awk '{ print $2-$3-'$MAGIC' }')
-	dd if=/dev/zero of=/var/bigfile bs=1M count=$DISKSPACE
+	busybox dd if=/dev/zero of=/var/bigfile bs=1M count=$DISKSPACE
 
 	# Ensure the update did not apply
 	if apply_update_sync -d $DELTAFILE ; then