qemu-iotests: Speed up make check-block

Using /tmp, which is usually mounted as tmpfs, the quick group can be
quicker.

On my laptop (Lenovo T430s with Fedora 20), this reduces the time from
50s to 30s.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/tests/qemu-iotests-quick.sh b/tests/qemu-iotests-quick.sh
index 12af731..0e554bb 100755
--- a/tests/qemu-iotests-quick.sh
+++ b/tests/qemu-iotests-quick.sh
@@ -3,6 +3,6 @@
 cd tests/qemu-iotests
 
 ret=0
-./check -T -qcow2 -g quick || ret=1
+TEST_DIR=${TEST_DIR:-/tmp/qemu-iotests-quick-$$} ./check -T -qcow2 -g quick || ret=1
 
 exit $ret