t/GtkRecentChooser.t: Forcibly emit GtkRecentManager::changed signal

Following Gtk+ commit 1070c5849e45433ad66c076e0bf692d936813a31, add_item
is even more asynchronous than before, and doesn't necessarily emit the
changed signal immediately, so GtkRecentChooser won't see the added
items.  Emit the changed signal manually to work around this.

https://bugzilla.gnome.org/show_bug.cgi?id=685865