Integration tests

[tsloughter]

I'd like to run full commands in docker, probably with shelltestrunner, to test they do as expected. The only thing we can't do is actual Erlang builds. So the docker image should have Erlang already installed in a place for beamup to recognize it and use.

Though there should be tests of the future features of erroring if wx or openssl aren't found, when an option like --warnings-as-errors is added to change future warnings of those missing into errors.

[tsloughter]

Partially done with shelltestrunner in #35