test: remove unused `util` imports

A number of tests in `test/parallel` were importing the `util` module
via `require()` but not using `util` for anything. This removes those
`require()` statements.

PR-URL: https://github.com/nodejs/node/pull/4397


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