Allow test suite to be disabled

There are some situations where the tests won't/can't work (for instance, on systems without dbus-daemon). Thus the tests don't have all their dependencies available. This is causing build failures:

tests/meson.build:6:0: ERROR: Command "/buildstream/carbonOS/pkgs/switcheroo-control.bst/tests/unittest_inspector.py /buildstream/carbonOS/pkgs/switcheroo-control.bst/tests/integration-test.py" failed with status 1.

Could you provide a way to disable the tests @ build time? For now, I'm just doing sed '/subdir..test/d' meson.build as a workaround