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 1253e7bfcbc80f88 for a similar fix on master.
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
