test: make common.js mandatory via linting rule

test/common.js contains code that detects global variable leaks.

This eslint rule checks that a module named `common` is loaded. It is
only applicable to files in the test directory. Tests that intentionally
leak variables can opt out with an eslint-disable comment.

PR-URL: https://github.com/nodejs/node/pull/3157


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>