support writing tests in C++ to test internals better

[shiftkey]

From #27

Need to add a test for dodgy non-null-terminated data, but I'm not sure the best way to actually get this into the registry in the first place, since REG.EXE is going to do it correctly. Can we add adjunct C++ programs to the test code somehow?

@Porges what your preferred C++ test framework for this sort of thing?

[Porges]

Nearly got something going but it crashes when I try to link it against the node functions and I don't know why yet 😁

[Porges]

Blergh; wasn't setting up the V8 platform abstraction before calling Initialize (why not use a normal class, eh?)

But CreateDefaultPlatform isn't exported from Node, so this can't link... Porges@6caf63f Will have to figure something out later.

[shiftkey]

I'm not entirely sure about this one. I'll do some digging and see if I can't spot a way out.