test: fix running child-process-uid-gid as root

This skips the child-test-uid-gid test when run as root.
Previously, the test failed if executed as root.

PR-URL: https://github.com/nodejs/node/pull/8864


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>