Integration tests with bitcoin in regtest mode

[afk11]

PRing this for your consideration - noticed some weird errors from bitcoind when you send an unknown RPC command, figured it could be interesting to run integration tests against bitcoind for certain cases. For now it's just a small test that the client gets something back from the server.

It's a separate issue for now, my motivation towards this kind of test is it allows us to test the library against a spectrum of bitcoind versions eventually. We could consider adding handling of bitcoind errors (named exceptions for the various error codes), and at least remain sure that we've implemented the right stuff.

Let me know what you think!

[afk11]

Any idea why the tests aren't running? @JoshuaEstes

[afk11]

Bump :) Any thoughts either way about this one? Just remembered it was open. I can add tests covering more cases if you'd like them, for now this PR is probably incomplete (a poc for now)

I actually made a package for spawning regtest nodes now, so can refactor out that stuff from here.

[JoshuaEstes]

Not sure why, it's something I'll need to look into once I get the time.