Oh, I believe it's a test....just not on that ship. More likely, it's a proverbial dry run to see what our response would be.
Also, it doesn't require any space in the hold to transport plutonium, which the NKs have as a byproduct of their reactor. They could be smuggling it anywhere on that ship. As another poster recommended, a radioactive sweep would be a wise thing to do.
Theoretically, they could also be picking up, instead of dropping off. Perhaps the NK fizzle test required a new supply of nuclear material in the form of enriched uranium. They could also be comparing notes, since the Taepo Dong and Shahab are both based on Scud technology.