Test loading references of assemblies loaded using LoadFrom and LoadFile.

There are three variations of the same two tests: LoadFromMain and LoadFileMain.

The differences are where we place the two additional assemblies Mid and Dep.
Mid references Dep.

samedir/
  Main, Mid and Dep are all in the same directory - both LoadFile and LoadFrom expected to work.
mainanddep/
  Main and Dep are in the base dir, Mid is in mid/ - both LoadFrom and LoadFrom expected to work
separatedir/
  Main is in the base dir, Mid and Dep are in mid/ - LoadFrom expected to work, LoadFile expected to fail.
