It is causing tests to timeout when libraries we are using
leak bus connections, and it turns out our tests run fine
without a session bus.
Backport of 1b265a7971171.
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
