189 run multiple test assemblies in single appdomain

Fixes #189, at least in my sandbox.

Includes "flushing" fix that @adamralph identified need for in #190.

Regarding why TD.Net doesn't see the error loading the second FakeItEasy.dll, I have  nothing. The lists of loaded paths and assemblies to load look the same for both NUnit and TD.Net. Switching the test (which some might argue could be over-specified - avoiding the exception is the real goal) to detect the "can't load FakeItEasy.IntegrationTests.External.dll" instead of "can't load FakeItEasy.dll" works under both runners. I've added that change and the Flush to the pull request. If nobody else has ideas, or a reason not to, perhaps we should take it?