test: allow globals to be whitelisted

This commit adds a function to test/common.js that allows
additional global variables to be whitelisted in a test.

PR-URL: https://github.com/nodejs/node/pull/7826
Reviewed-By: James M Snell <jasnell@gmail.com>

test/common.js

@@ -345,6 +345,11 @@ if (global.Symbol) {
   knownGlobals.push(Symbol);
 }
 
+function allowGlobals(...whitelist) {
+  knownGlobals = knownGlobals.concat(whitelist);
+}
+exports.allowGlobals = allowGlobals;
+
 function leakedGlobals() {
   var leaked = [];