test: enable linting for tests

Enable linting for the test directory. A number of changes was made so
all tests conform the current rules used by lib and src directories. The
only exception for tests is that unreachable (dead) code is allowed.

test-fs-non-number-arguments-throw had to be excluded from the changes
because of a weird issue on Windows CI.

PR-URL: https://github.com/nodejs/io.js/pull/1721


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>