test: preserve env in test cases

Allows env vars to be passed through to child processes. This is needed
for things like NODE_TEST_DIR or LD_LIBRARY_PATH if testing the shared
library.

Backport-PR-URL: https://github.com/nodejs/node/pull/18883
PR-URL: https://github.com/nodejs/node/pull/14822
Refs: https://github.com/nodejs/node/pull/13390


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>