Add unit testing for Lust programs

[0xekez]

At the moment there is no way to test that errors are being displayed properly because they print to stdout and don't leave much of a trace. As time goes on and there is more need for this sort of thing we should consider adding some sort of btest like unit test framework so that we can compare the output of lust programs to a baseline.

[0xekez]

In tests/main.lisp there is some work on this. Once there is better IO support it would be nice to move towards something more like Zeek's btest.