Trunk 4446: Support deriving auto_expire_date from duration and duration units for Drug Orders

Changes for ticket https://issues.openmrs.org/browse/TRUNK-4446
But Context.getDateFormat uses the default locale which is always 'en_GB' by default so in test data you should be fine. But i guess it is not a big deal since it is in unit tests, you can leave it the way it is.