This allows two paths:

1. If you have the YAML PECL extension, then that will be used.
   To get the best output, you can install it.
2. If you don't, then it'll output JSON. JSON is considered valid YAML.
   It's not clear that everyone reading the document will handle
   the full range YAML (incl JSON), but... we don't actually have any
   listeners that care to parse the TAP output...