Validate files in Authors folder are present in csproj

[Cheesebaron]

Is your feature request related to a problem? Please describe.
Sometimes people just add the class to the authors folder without adding to the csproj file.

Describe the solution you'd like
While we are still not on the new style csproj files, we should consider adding some validation to the test suite that ensures tests blow up if csproj entry is missing for the file

Describe alternatives you've considered
Upgrade to ASP.NET Core and new SDK style csproj files

[jamesmontemagno]

I think we should do some ASP.NET Core migration for 3.0 perhaps :)