Implement support for unit testing

[kendalharland]

A simple start would be an Oba module with a small framework for writing unit tests which are run as normal programs. If a test fails, it ensures the program's exit code is nonzero. If all tests pass, the exit code should be zero unless the user's code explicitly sets something else.

A nice followup might involve adding a test subcommand to the interpreter if we want to add more complexity like filtering tests by name, changing the output format, etc..