test: remove unnecessary assignments

test: remove unnecessary assignments

common.js needs to be loaded in all tests so that there is checking
for variable leaks and possibly other things. However, it does not
need to be assigned to a variable if nothing in common.js is referred
to elsewhere in the test.

The main tradeoff for this bit of code churn is that it gets the code
base most of the way to being able to enable the no-unused-vars rule in
eslint.

(The non-tooling benefit is that it lessens cognitive load when reading
tests as it is an immediate indication that none of the functions or
properties in common.js will be used by the test.)

PR-URL: https://github.com/nodejs/node/pull/4563
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>