Running unit tests via Ant - loading property files

I have my unit tests running fine in eclipse with AllTestSuite
having followed http://groovy.codehaus.org/Unit+Testing

I am trying to set them up to run via Ant. I have some properties files stored in the jar that are loaded in my code using, eg.
      InputStream is = getClass().getResourceAsStream('/properties/myconfig.groovy')

When this code is run via the <junit> tag in ant, the result of this line, is, evaluates to null.

How do I deal with this?

Thanks, Tom.