Visual Studio knows nothing about an Arduino project. Why would you be trying to reference it from the other project. Short answer is that you are not understanding how to use Visual Studio and / or Arduino. Are you using it with some plugin for visual studio? If you are more than likely it does not support being part of a multi solution (at least that has been my experience with them).
Long answer is give some more details about what you are trying to do, including code from both projects.
Update: Sorry but this is not an Arduino topic. You are fundamentally incorrect in your understanding of how compilers and linkers work and what they are for. As are the other answers. You can not reference a project compiled for an Arduino, from a project compiled for Windows, period. I ask you to explain what you were trying to do and I would give you some idea how to accomplish that, but you did not answer. Yes you can use Visual Studio to compile your Arduino project, I do it every day. But I do not 'reference' it from another project on Windows. Do you have any idea what a reference is?
update 2: You can not create a test suite for an Arduino project, at least not in the traditional way. You can not even debug an Arduino project in the traditional way. Arduino is for prototyping, and very basic beginner stuff, and nothing more. What you are asking about is more traditional development, and the Arduino is not for that (and does not do it very well).
Create 2 separate solutions. One with only your visual micro project for the arduino, and another separate solution with a regular C++ or C# project, not a test project or anything special, just a plain console, or winforms, or WPF project. The 2 pieces of code must be run at the same time (F5), and probably connect to each other via the serial port, but it depends on what you are doing.
This IS NOT testing in a traditional sense. This is test running a project. The traditional sense of testing is to test isolated portions of code, which is different, and again, can not be done with an Arduino and visual studio. Since the Arduino only remotely provides 2 wire debugging, and I have never heard anyone actually get it to work at all, you can NOT do any traditional debugging or testing with an Arduino. Instead you are limited to beginner hacks (serial.print).