test: remove envPlus, use Object.assign everywhere

PR-URL: https://github.com/nodejs/node/pull/14845
Fixes: https://github.com/nodejs/node/issues/14823
Refs: https://github.com/nodejs/node/pull/14822


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>