How to start plugins for (Unit)-Testing in C# /.Net

I'm using BDD/MSpec(basicalley extended unit-test) to extend an already existing plugin. In order to test my methods, I have to start programm that is extended by the plugin and the plugin itself before I can conduct my unit-tests.

Any ideas how to do that?

I suppose

new MainProgrammInstance();
new PluginInstance();
...regular uni-tests

won't do the trick.