Adding Tests To Protobuild

This PR converts over the test projects to use protobuild.

With this change the tests are now in the same solution as the framework and content pipeline projects.  Hopefully this will encourage people to work more on the tests.

Note the XNA version of the tests were not converted to use Protobuild.  To run them you must manually add the `MonoGame.Tests.XNA.csproj` into the solution.  We'll look to fix this in the future.