After trying to build my unit test project I noticed that some of the private accessors in my unit tests were no longer referenced properly. I tried to regenerate a unit test and came across the following error:
"The following error was encountered while reading module 'Microsoft.SharePoint': Assembly reference not resolved: Microsoft.Internal.Mime, Version=18.104.22.168, Culture=neutral, PublicKeyToken=31bf3856ad364e35."
When I copied the dll of the project throwing the error to the GAC the build error disappeared. I removed the dll and the error was back. I then tried to remove the Microsoft.SharePoint reference from my project and add it back. Still no luck.
My final step was to delete the project file from the solution and re-create the project with a fresh project file. Problem solved.