benchmark: `util._extend` vs `object.assign`

To copy the values of all enumerable own properties from-
a source object to a target object, node still use-
`util._extend`, though newer standard `Object.assign`
is available. This is because `util._extend` is found to
be faster than `Object.assign`. This benchmark test is
to keep track of how performance compare.

PR-URL: https://github.com/nodejs/node/pull/7255


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>