test: robust handling of env for npm-test-install

Currently we are overwriting the entire env object of the child-process
spawned in `npm-test-install`. This commit alternatively clones the
`process.env` object and modifies it with the neccessary changes before
passing it the the spawned process.

Fixes: https://github.com/nodejs/node/issues/6736

PR-URL: https://github.com/nodejs/node/pull/6797


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>