nunit: Our public ConcurrentQueue causes type conflicts

If you try to use NUnit on net35 with another library such as AsyncBridge which ships a ConcurrentQueue polyfill, the project won’t compile because of the type collision between the two dependencies.

This began in 3.10. We should make sure all our polyfill classes are internal since they are sharing the System namespace.

The same problems occur with NUnit.System.Linq, and System.Action etc, but I’m not sure we can do anything about that because it has shipped with NUnit for so long. If we kept it in the nupkg copying to the test project output folder but kept the test project from referencing it, it would solve the problem but it might also break people who have come to rely on NUnit.System.Linq always coming with NUnit.