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>

test/parallel/test-https-truncate.js

@@ -3,7 +3,7 @@ var common = require('../common');
 var assert = require('assert');
 
 if (!common.hasCrypto) {
-  console.log('1..0 # Skipped: missing crypto');
+  common.skip('missing crypto');
   return;
 }
 var https = require('https');