test: remove unused modules

Many tests use require() to import modules that subsequently never gets
used. This removes those imports and, in a few cases, removes other
unused variables from tests.

PR-URL: https://github.com/nodejs/node/pull/4475


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>