tests: Add regression test for simple scaling

Add regression test for simple scaling. This is the kind of XML file
you would expect to get if all you want to do is scale an image with
GEGL.

make distcheck has been confirmed to still pass.

tests/compositions/simple-scale.xml

+<?xml version='1.0' encoding='UTF-8'?>
+<gegl>
+  <node operation='gegl:scale'>
+    <params>
+      <param name='filter'>cubic</param>
+      <param name='x'>0.5</param>
+      <param name='y'>0.5</param>
+    </params>
+  </node>
+  <node operation='gegl:load'>
+    <params>
+      <param name="path">data/grid.png</param>
+    </params>
+  </node>
+</gegl>