test: use npm sandbox in test-npm-install

npm should run in a sandbox to avoid unwanted interactions. Without
this change, npm would read the userconfig file $HOME/.npmrc which may
contain configs that break this test.

Fixes: https://github.com/nodejs/node/issues/9074
PR-URL: https://github.com/nodejs/node/pull/9079


Reviewed-By: Jeremiah Senkpiel <Fishrock123@rocketmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>