tests: Extract images to /var/tmp, not /tmp

Test VMs don't have enough memory (/tmp is a tmpfs).

tests/cli/test_compose_tar.sh

@@ -48,7 +48,7 @@ rlJournalStart
 
     rlPhaseStartTest "Verify tar image with systemd-nspawn"
         if [ -f /usr/bin/systemd-nspawn ]; then
-            NSPAWN_DIR=`mktemp -d /tmp/nspawn.XXXX`
+            NSPAWN_DIR=`mktemp -d /var/tmp/nspawn.XXXX`
             rlRun -t -c "tar -xJvf $IMAGE -C $NSPAWN_DIR"
 
             # verify we can run a container with this image