Add GTestDBus object

This is a helper to write unit tests using a private dbus-daemon.

session_bus_up/down() are now just wrappers around a GTestDBus singleton.

https://bugzilla.gnome.org/show_bug.cgi?id=672985

docs/reference/gio/gio-docs.xml

@@ -230,6 +230,10 @@
         <xi:include href="xml/gdbus-codegen.xml"/>
         <xi:include href="gresource.xml"/>
     </chapter>
+    <chapter id="testing">
+        <title>GIO Testing</title>
+        <xi:include href="xml/gtestdbus.xml"/>
+    </chapter>
   </part>
 
   <part id="migrating">

docs/reference/gio/gio-sections.txt

@@ -3870,3 +3870,21 @@ g_resource_flags_get_type
 g_resource_lookup_flags_get_type
 g_resource_error_quark
 </SECTION>
+
+<SECTION>
+<FILE>gtestdbus</FILE>
+<TITLE>GTestDBus</TITLE>
+GTestDBus
+GTestDBusFlags
+g_test_dbus_new
+g_test_dbus_get_flags
+g_test_dbus_get_bus_address
+g_test_dbus_add_service_dir
+g_test_dbus_up
+g_test_dbus_stop
+g_test_dbus_down
+g_test_dbus_unset
+<SUBSECTION Private>
+g_test_dbus_get_type
+g_test_dbus_flags_get_type
+</SECTION>

docs/reference/gio/gio.types

@@ -133,3 +133,5 @@ g_menu_attribute_iter_get_type
 g_menu_link_iter_get_type
 g_menu_get_type
 g_menu_item_get_type
+g_test_dbus_get_type
+g_test_dbus_flags_get_type