tools: remove NODE_PATH from environment for tests

Unset NODE_PATH environment variable when running tests.

On Ubuntu 16.04, some users experience test failures due to internal
libraries being installed in /usr/lib/nodejs/internal and NODE_PATH
including /usr/lib/nodejs. Tests that expect internal libraries to be
off limits without the --expose-internals flag will fail in this
situation. Currently, those tests are test/parallel/test-repl.js and
test/parallel/test-internal-modules.js.

This situation seems to (probably) be caused by some
not-entirely-uncommon package that gets installed.

Regardless, tests should ignore the user's NODE_PATH. (NODE_PATH is
tested in test/parallel/test-module-globalpaths-nodepath.js and
test/parallel/test-require-dot.js.)

PR-URL: https://github.com/nodejs/node/pull/11612


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Refs: https://twitter.com/trott/status/8357293969...