Add a reftest for multiple font families

This tests that listing multiple font
families in the css font-family property
works as expected.

testsuite/reftests/label-fonts.ref.ui

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <object class="GtkWindow" id="window1">
+    <property name="type">popup</property>
+    <child>
+      <object class="GtkLabel" id="label3">
+        <property name="name">reference</property>
+        <property name="label" translatable="yes">abcdefg</property>
+      </object>
+    </child>
+  </object>
+</interface>