patman: hide the 'test' command unless test data is available

Some tests would fail when the test data is not available, so it
doesn't make much sense to expose the action when patman is running
outside of the u-boot git checkout.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>