Add negative test cases to commandParser unit tests

[davidmisiak]

Currently, we only test successful parsing attempts, negative tests (ie. invalid CLI arguments / argument combinations) should be added.

This requires setting exit_on_error=false in argparse when running tests (so that an exception is thrown instead of terminating the process).

Particularly, we should add negative tests of mutually exclusive flags, added in #109. Note that this may be a bit difficult - I suspect that this line in argparse causes process termination even when exit_on_error is false.

Some related discussion here #109 (comment).