tools: expose skip output to test runner

In the TAP protocol, skips are flagged as ok. Expose more
information so we can understand if the test was skipped or not.

PR-URL: https://github.com/nodejs/io.js/pull/2130


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>