Remove output from tests

`Tabber` accepts an `:output` option, which points to `$stdout` by default.
Tests set this option to a `StringIO` object instead.

If you like the output for acceptance testing, feel free to close this. But a high-level test that makes explicit assertions about the format of the output would be better imo. In fact, I might just write that test :)

I'll rebase if https://github.com/venmo/synx/pull/36 gets merged first and you like this change.