Refactor the CLI to make it more testable and eliminate multhreading in its unit tests

[dlurton]

The unit tests for the REPL are multithreaded. I'm guessing this is because the design of the CLI isn't easily testable. Unfortunately, there's a mysterious race condition in this code that showed up recently that's causing our travis CI builds to fail randomly. This build, for example

While doing this, we should also consider these other two issues #112, #255, which together strongly suggest to me that the CLI should be refactored to use an ncurses type library for the JVM. (Perhaps lanterna could be wrangled for this purpose? This is just an idea, I haven't seriously looked to see if lanterna is a good fit for this. Another alternative is jline3 which seems targeted to this purpose.)

EDIT: #267 disables this test for the time being.