Hi. I did a test Library to see if it works in Unity and it doesn't I get this error: Internal compiler error. See the console log for more information. output was: Unhandled Exception: System.TypeLoadException: Could not load type 'System.Runtime.Versioning.TargetFrameworkAttribute' from assembly 'LibaryTestApp'. It's just a simple dll containing two simple classes.
What is in the test classes? If they use code in libraries not supplied with Unity then you may also need to include other DLLs in the project.
This is old I know, but perhaps you are building the Library in Visual Studio and you project settings are for .Net 4.0? I had the same error and changing to .Net 2.0 fixed my problems.