How do the *-external* files differ? How should they be used?
The -external tests can be used if for some reason you are not using the built-in ARegister, DRegister and PC components. In that case the normal tests will fail to load because they require a special testing interface to those built-in components.
If you are using the built-in components, you should use the regular tests. They give you additional information about what's happening inside your CPU and Computer.