The test was using a nonexisting GtkActionBar::spacing property.
<object class="GtkBox">
<property name="visible">True</property>
<property name="can_focus">False</property>
+ <property name="spacing">6</property>
<child>
<object class="GtkButton">
<property name="visible">True</property>
<object class="GtkActionBar">
<property name="visible">True</property>
<property name="can_focus">False</property>
- <property name="spacing">0</property>
<child>
<object class="GtkButton">
<property name="visible">True</property>