Stop trying to deal with "theoretical possibilities".
We can't possibly continue to be a faithful GActionGroup implementation
across dispose because dispose has a side effect of removing everyone's
signal handlers.
The code that we ran after the dispose chainup to do all of the fancy
signal emulation was therefore dead. The test that aimed to verify this
was buggy itself due to an uninitialised variable, so really, it never
worked at all.
We keep the re-ordering of the chainup from the original commit to avoid having
trouble with GtkActionMuxer and keep the checks in place that will prevent an
outright segfault in the case that someone else tries to use the interface
post-dispose.
https://bugzilla.gnome.org/show_bug.cgi?id=722189
Make sure that the tests don't access the host's session bus or
installed gsettings schemas.
Also disable tests for some classes that leak a connection to the
session bus.
https://bugzilla.gnome.org/show_bug.cgi?id=711715
So far, this is just supposed to be gdk_cairo_set_source_pixbuf().
Note that this is usually not an API guarantee but courtesy to
applications that used these APIs without a gtk_init() call.
https://bugzilla.gnome.org/show_bug.cgi?id=708547
