Create abstract class for command unit tests

aesh-extensions already has AeshTestCommons, written by heliofrota. That's a great start, but I'm having some issues with it. Most notably, there appears to be some timing issues when debugging in the IDE. If a breakpoint occurs in the middle of a succession of commands (using #pushToOutput), the break screws up the processing of all proceeding commands. (Ie, if run without debugging, you get entirely different results).

I understand this is due to the multi-threaded nature of Aesh. If nothing else, it might be a good idea for us to come up with a concise abstract test class, as well as a set of docs, to help facilitate unit tests on custom commands.