a11y tests: Add a tests for menu buttons with popovers

This shows that popovers currently lack accessibility.

testsuite/a11y/Makefile.am

@@ -75,6 +75,7 @@ testdata = \
 	menu.ui menu.txt \
 	menubutton.ui menubutton.txt \
 	menubutton2.ui menubutton2.txt \
+	menubutton3.ui menubutton3.txt \
 	combos.ui combos.txt \
 	listbox.ui listbox.txt \
 	stack.ui stack.txt \

testsuite/a11y/menubutton3.txt

@@ -0,0 +1,24 @@
+window1
+  "frame"
+  index: 0
+  state: enabled resizable sensitive showing visible
+  toolkit: gtk
+  <AtkComponent>
+  layer: window
+  alpha: 1
+  menubutton
+    "toggle button"
+    parent: window1
+    index: 0
+    name: Menu
+    state: enabled focusable sensitive showing visible
+    toolkit: gtk
+    <AtkComponent>
+    layer: widget
+    alpha: 1
+    <AtkImage>
+    image size: -1 x -1
+    image description: (null)
+    <AtkAction>
+    action 0 name: click
+    action 0 description: Clicks the button

testsuite/a11y/menubutton3.ui

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <!-- interface-requires gtk+ 3.0 -->
+  <menu id="menu">
+    <section>
+      <item>
+        <attribute name="label">New</attribute>
+      </item>
+    </section>
+  </menu>
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <child>
+      <object class="GtkMenuButton" id="menubutton">
+        <property name="visible">True</property>
+        <property name="can_focus">True</property>
+        <property name="menu-model">menu</property>
+        <property name="use-popover">True</property>
+      </object>
+    </child>
+  </object>
+</interface>