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

gio/Makefile.am

@@ -65,6 +65,7 @@ gdbus_headers = 			\
 	gdbusobjectmanager.h		\
 	gdbusobjectmanagerclient.h	\
 	gdbusobjectmanagerserver.h	\
+	gtestdbus.h			\
 	$(NULL)
 
 gdbus_sources = 							\
@@ -95,6 +96,7 @@ gdbus_sources = 							\
 	gdbusobjectmanagerclient.h	gdbusobjectmanagerclient.c	\
 	gdbusobjectmanagerserver.h	gdbusobjectmanagerserver.c	\
 	gdbus-tests.h			gdbus-tests.c			\
+	gtestdbus.h			gtestdbus.c			\
 	$(NULL)
 
 settings_headers = \