test: abstract skip functionality to common

The tap skipping output is so prevalent yet obscure in nature that we
ought to move it into it's own function in test/common.js

PR-URL: https://github.com/nodejs/node/pull/6697


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>