Fixing tests

Because the InMemory tests were in the same assembly as the TempFile tests they were preloading the assemblies into the AppDomain when running all the tests.

This has the side effect of making the TempFile tests look like they're passing when they're not.

By moving the TempFile tests into their own assembly NUnit uses a different AppDomain for each assembly under test.

**NOTE** Don't close this yet, I'm still working on it. I just wanted to put it here to discuss the issues.