test: linting for buffer-free-callback test

Test added in d1f24044 does not pass linting rule added in 3de353b5.
Fixed in this commit. `common` module required in all tests except
those that intentionally leak variables.

PR-URL: https://github.com/nodejs/node/pull/3230
Fixes: https://github.com/nodejs/node/issues/3229
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

test/addons/buffer-free-callback/test.js

@@ -1,6 +1,7 @@
 'use strict';
 // Flags: --expose-gc
 
+require('../../common');
 var assert = require('assert');
 var binding = require('./build/Release/binding');
 var buf = binding.alloc();