Figure out how to implement a test task

[arrdem]

At present, the compile target is really magical and extremely privileged because of how much context the roll function needs to behave as desired.

The goal of this ticket is to be able to deliver a

$ ./kat test [target ...]

which is generic and which has a clear pattern for integrating with the same change detection used by compile.

[arrdem]

At least a working clojure-test target is the primary blocker on (next RELEASE). It may be worth taking a look back at other similar systems like Pants and Buck to figure out how they conceive of user-level tasks which interact with build products and whether there's something which could be cribbed there.

As I recall Pants was pretty egalitarian about what tasks could do - although I don't remember how tests worked for invalidation purposes - Buck is much more ah firm about what the tasks are and how they behave. I seem to recall that bolting a repl task onto Buck wasn't super trivial.