I will repeat the advice I gave you in my answer to your other question by suggesting that you look at using FireBreath for this plugin; it already has the wrappers to make NPObjects work well and as an added bonus on windows you can use your plugin on IE also.
That said, writing unit tests for a NPAPI plugin is fairly difficult. Depending on how your code is written you could "fake out" a few of the important functions -- such as the memory management functions NPN_MemAlloc and NPN_MemFree -- and then use those to write unit tests, or you could write automated tests that use a web browser.
FireBreath includes a NPAPIHost project that was designed to assist with unit testing NPAPI plugins, but it has not been fully completed. Still, you could use it as a starting point if you want.
Honestly, though, I would just use FireBreath and then you can focus on your own code instead of trying to write and maintain your own abstraction for the scripting system.