test,tools: enable linting for undefined vars

The test directory had linting for undefined variables disabled. It is
enabled everywhere else in the code base. Let's disable the fule for
individual lines in the handful of tests that use undefined variables.

PR-URL: https://github.com/nodejs/node/pull/6255


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>