Write tests

[mitchellwrosen]

This app is very IO-heavy and side-effecty. Would be nice to refactor it to write some pure tests, be able to feed in pure (mocked) filesystem events, and stuff like that.

Some simple high-level properties to test:

• The same job should not be enqueued twice (so e.g. saving multiple times in succession will not each trigger a separate unit test run)
• A job identical to the one currently running should cause it to be canceled and restarted

Then there's all the untested IO ugliness:

• Filesystem oddities (dead symlinks, permission errors, etc)
• Failed spawned processes, or rogue processes that don't even respond to SIGINT (not sure if Haskell can do anything about these at all)
• Async exceptions handling