reftests: Fix unresolvable.ui test

- Make sure we draw something
- Don't use -gtk-gradient() but linear-gradient instead

testsuite/reftests/unresolvable.css

@@ -1,12 +1,13 @@
 @import "reset-to-defaults.css";
 
 window {
+  background-color: black;
+}
+
+box {
   background-color: @doesntexist;
   box-shadow: @nonexistant 5px 5px;
-  border-image: -gtk-gradient(linear,
+  border-image: linear-gradient(to top, @notaavailable, @whatever);
-                              left top, left bottom,
-                              from(@notaavailable),
-                              to(@whatever)) 2;
 }
 
 #reference {

testsuite/reftests/unresolvable.ref.ui

@@ -1,11 +1,15 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
   <!-- interface-requires gtk+ 3.0 -->
-  <object class="GtkWindow" id="window1">
+  <object class="GtkWindow">
-    <property name="width_request">10</property>
-    <property name="height_request">10</property>
     <property name="can_focus">False</property>
     <property name="type">popup</property>
-    <property name="name">reference</property>
+    <child>
+      <object class="GtkBox">
+        <property name="name">reference</property>
+        <property name="width_request">10</property>
+        <property name="height_request">10</property>
+      </object>
+    </child>
   </object>
 </interface>

testsuite/reftests/unresolvable.ui

@@ -1,13 +1,14 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
   <!-- interface-requires gtk+ 3.0 -->
-  <object class="GtkWindow" id="window1">
+  <object class="GtkWindow">
-    <property name="width_request">10</property>
-    <property name="height_request">10</property>
     <property name="can_focus">False</property>
     <property name="type">popup</property>
     <child>
-      <placeholder/>
+      <object class="GtkBox">
+        <property name="width_request">10</property>
+        <property name="height_request">10</property>
+      </object>
     </child>
   </object>
 </interface>