Include a test of the z_sort function

tests/data/refs/sis__e83b8e714fc4eddd89880961181b861c.out

@@ -0,0 +1,13 @@
+<!-- Created with Inkscape (http://www.inkscape.org/) --><svg xmlns="http://www.w3.org/2000/svg" xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd" xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:cc="http://web.resource.org/cc/" xmlns:dc="http://purl.org/dc/elements/1.1/" width="793.7007874" height="1122.519685">
+  <defs/>
+  <sodipodi:namedview id="base" pagecolor="#ffffff" bordercolor="#666666" borderopacity="1.0" inkscape:pageopacity="0.0" inkscape:pageshadow="2" inkscape:zoom="0.35" inkscape:cx="375" inkscape:cy="520" inkscape:document-units="px" inkscape:current-layer="layer1"/>
+  <metadata>
+    <rdf:RDF>
+      <cc:Work rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type rdf:resource="http://purl.org/dc/dcmitype/StillImage"/>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <g inkscape:label="Layer 1" inkscape:groupmode="layer" id="layer1"><rect x="40" y="40" width="100" height="60" style="stroke:black;fill:tan;opacity:0.9"/><rect x="30" y="30" width="100" height="60" style="stroke:black;fill:mediumseagreen;opacity:0.9"/><rect x="20" y="20" width="100" height="60" style="stroke:black;fill:mediumslateblue;opacity:0.9"/><rect x="10" y="10" width="100" height="60" style="stroke:black;fill:maroon;opacity:0.9"/><rect x="0" y="0" width="100" height="60" style="stroke:black;fill:beige;opacity:0.9"/></g>
+</svg>

tests/test_simple_inkscape_scripting.py

@@ -322,7 +322,12 @@ class SimpInkScrTestModExObjs(CustomComparisonMixin,
         ("--program=%s\nboxes[-1].z_order('bottom')" % z_order,),
         ("--program=%s\nboxes[2].z_order('raise')" % z_order,),
         ("--program=%s\nboxes[3].z_order('lower', 2)" % z_order,),
-        ("--program=%s\nboxes[1].z_order('to', 3)" % z_order,)
+        ("--program=%s\nboxes[1].z_order('to', 3)" % z_order,),
+        (f'''--program=
+{z_order}
+for obj in z_sort(boxes):
+    obj.z_order('bottom')
+''',)
     ]
     compare_file = 'svg/default-inkscape-SVG.svg'