a11y tests: Fix settings that affect a11y output

It would be nicer if we could have the tests specify what environment
their expected output was created in, then we could test multiple
scenarios. For now, just fix the setting to avoid test failures.

testsuite/a11y/accessibility-dump.c

@@ -884,12 +884,26 @@ parse_command_line (int *argc, char ***argv)
   return TRUE;
 }
 
+static void
+fix_settings (void)
+{
+  /* Some settings can affect the output of the accessibility tests,
+   * so we take some steps to isolate us from the ambient environment.
+   */
+
+  g_object_set (gtk_settings_get_default (),
+                "gtk-dialogs-use-header", TRUE,
+                NULL);
+}
+
 int
 main (int argc, char **argv)
 {
   if (!parse_command_line (&argc, &argv))
     return 1;
 
+  fix_settings ();
+
   if (argc < 2)
     {
       const char *basedir;