test: formatting skip messages for TAP parsing

This patch makes the skip messages consistent so that the TAP plugin
in CI can parse the messages properly. The format will be

    1..0 # Skipped: [Actual reason why the test is skipped]

PR-URL: https://github.com/nodejs/io.js/pull/2109


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>