I guess they are testing to see how fuel level changes affect the flight characteristics and centre of gravity etc. Instead of flying around for hours just to burn fuel, it would be more efficient to just rapidly dump it and get straight to business.
I would imagine that this would be a necessary step before they clear her for any long distance flights, so if the plane suddenly behaves unexpectedly as fuel levels drop, they have a runway right under them. That is a lot safer than if they experienced problems during a long range test/ferry flight hundreds of miles from a suitable airport.
This could mean they are laying the foundations for moving her to more remote test centers if this is the case.
Personally I think high intensity testing is still pretty far away and might not even happen until 2002 comes out.