LINK
I agree that N.Korea is testing international response. Already on several news sources like the BBC, the one I linked above and the one by provided by rdret1, they specifically mention these attacks come after a visit by an American scientist to their uranium enrichment facility.
Sounds like a twisted PR stunt to show the world the new leader can play in the big leagues.
THIS article seems to suggest they are trying to put the U.S. in a politically sour position, while they shell south korea. "I remove some fuel rods, you tell the world you don't have anything against me."