testsuite: Replace XPM file

The XPM loader in GdkPixbuf is optional, so we should avoid using it.

See: #7143

testsuite/reftests/image-load-from-file.css

@@ -5,5 +5,5 @@
 }
 
 #reference2 {
-  background: url("marble.xpm") no-repeat;
+  background: url("marble.png") no-repeat;
 }

testsuite/reftests/image-load-from-file.ui

@@ -25,7 +25,7 @@
           <object class="GtkImage" id="image2">
             <property name="visible">True</property>
             <property name="can_focus">False</property>
-            <property name="file">marble.xpm</property>
+            <property name="file">marble.png</property>
           </object>
           <packing>
             <property name="expand">False</property>

testsuite/reftests/meson.build

@@ -305,7 +305,7 @@ test_data = [
   'link-coloring.css',
   'link-coloring.ref.ui',
   'link-coloring.ui',
-  'marble.xpm',
+  'marble.png',
   'messagedialog-secondarytext.ui',
   'messagedialog-secondarytext.ref.ui',
   'misc-alignment.css',