Under autotools the compiled schemas are in the build directory and with
meson they are in the root build dir. Avoid changing the autotools build for
now and add a special GTK_TEST_MESON env var which we can use to differentiate
the two.
See 1253e7bfcb for a similar fix on master.
It seems that alternate implementations of GtkFileChooserWidget
never materialized. The split between GtkFileChooserWidget and
GtkFileChooserDefault is awkward. The immediate problem is that
it makes it difficult to document the keybinding signals. So it
makes sense to drop the abstraction and just have one thing.
https://bugzilla.gnome.org/show_bug.cgi?id=723157
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
