With autotools the schemas were compiled into each test suite directory
and the tests set GSETTINGS_SCHEMA_DIR to the test build directory.
With meson's gnome.compile_schemas() we can not define a target directory
so just make sure it is built in the gtk directory and set GSETTINGS_SCHEMA_DIR
to the gtk build directory when running the tests.
This makes the gtk+:gtk suite pass when no gtk is installed on the system.
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
