tests: Sort size summary by image size and rootfs size

Sort the table of image sizes by rootfs size after having sorted by
image size so that the smaller rootfs images appear before larger ones
when the image size is the same.

Fixes #178.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

tests/test_images.sh

@@ -120,7 +120,7 @@ show_stats()
 		"Type" \
 		"Name"
 
-	sort -k1,1n "$tmpfile"
+	sort -k1,1n -k3,3n "$tmpfile"
 
 	rm -f "${tmpfile}"
 }