test: remove common.inspect()

common.inspect() is just util.inspect(). common copies every property
from util but this is the only one that gets used and it only gets used
in three places. Well, four, but the fourth is removed in a pending PR.

This commit removes it. Subsequently, the "copy util to common"
part of `common` can be removed altogether.

PR-URL: https://github.com/nodejs/node/pull/3257


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>