Dummy callbacks for tests

[aahadley-anchorage]

If external callbacks are specified, the build can fail at link time since these functions aren't defined yet. Give the linker some definitions to make it happy.

[real-or-random]

Hm, that's an issue indeed.

If you use the external callbacks, then you probably do because you don't have printf or abort on your platform. In tests, we use these function anyway, so I think this change is fine. But wouldn't it be simpler to #undef USE_EXTERNAL_CALLBACKS?

Ideally, we'd move into the other direction and make it possible to run the tests without printf. But that's more complicated.

I wonder if could we just abuse the external callbacks for exactly that purpose? Instead of making CHECK call abort, we could make this macro reset the default callback and then invoke it. If you don't have printf or abort, you'd be able provide a different external callback... Except I don't know how to print the seed then or the final success message. Maybe it's just cleaner to let the user provide some print function...

[real-or-random]

Closing in favor of #1306