reftests: Add test for GtkImage use-fallback property

Fallback seems to be working in the GtkIconTheme test suite, but fails
in GtkImage itself.

This is a test for a bug in the Bluetooth settings. An icon named
"phone-apple-iphone" should fallback to "phone" if the
gnome-icon-theme-extras package isn't installed.

testsuite/reftests/Makefile.am

@@ -265,6 +265,8 @@ testdata = \
 	gtk-icontheme-sizing.css \
 	gtk-icontheme-sizing.ref.ui \
 	gtk-icontheme-sizing.ui \
+	gtk-image.ui \
+	gtk-image.ref.ui \
 	icon-shadow-no-transform.css \
 	icon-shadow-no-transform.ref.ui \
 	icon-shadow-no-transform.ui \

testsuite/reftests/gtk-image.ref.ui

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Generated with glade 3.19.0 -->
+<interface>
+  <requires lib="gtk+" version="3.10"/>
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <property name="type">popup</property>
+    <child>
+      <object class="GtkImage" id="image1">
+        <property name="visible">True</property>
+        <property name="can_focus">False</property>
+        <property name="icon_name">edit-delete</property>
+        <property name="use_fallback">True</property>
+      </object>
+    </child>
+  </object>
+</interface>

testsuite/reftests/gtk-image.ui

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Generated with glade 3.19.0 -->
+<interface>
+  <requires lib="gtk+" version="3.10"/>
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <property name="type">popup</property>
+    <child>
+      <object class="GtkImage" id="image1">
+        <property name="visible">True</property>
+        <property name="can_focus">False</property>
+        <property name="icon_name">edit-delete-lolwtfbbq</property>
+        <property name="use_fallback">True</property>
+      </object>
+    </child>
+  </object>
+</interface>