Jesse Liberty has posted on how to make the Silverlight Unit Tests work on Windows Phone.
Source: Jesse's Blog
The latest release of the Silverlight Unit Tests comes with the Silverlight Toolkit, and it targeted at Silverlight 4. Windows Phone is based on an enhanced version of Silverlight 3 and cannot use these DLLs.
Fortunately, Jeff Willcox has made the right DLLs available on his web site.