Tests: add session_bus_run() and use it where possible

This is to avoid having again the subtil bug in dbus-appinfo.c:
session_bus_down() was called before g_test_run() so the test was
running on the user's dbus session.

https://bugzilla.gnome.org/show_bug.cgi?id=697348

gio/tests/gdbus-sessionbus.h

@@ -30,6 +30,7 @@ G_BEGIN_DECLS
 void session_bus_up   (void);
 void session_bus_stop (void);
 void session_bus_down (void);
+gint session_bus_run (void);
 
 G_END_DECLS