test: remove unused assert module imports

Many test modules load assert but do not use it. This change removes
those instances.

It also removes a handful of other unused variables when they were
nearby.

PR-URL: https://github.com/nodejs/node/pull/4438


Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>