Launch unit tests within a single app domain

[Eric Wallace]

As discussed on the board at https://groups.google.com/forum/#!topic/nunit-discuss/NVMF6XcuAmk, when calling into a C++/CLI method from a C# unit test, an exception occurs when trying to access protected memory across domains when each assembly is assigned to a separate app domain. The option to load all assemblies into one app domain should exist.

Bug submitted as per Charlie Poole's request.