为什么在引用来自同一解决方案的另一个项目时，会得到一个FileNotFound异常?

I am learning how to use NUnit. I have my main project in it's solution, and created a separate project in that same solution which will hold my unit tests, with it's own namespace. From that project I add a reference to the main project and add a I am learning how to use NUnit. I have my main