test: fix child-process-uid-gid on Windows

The process.getuid method does not exist on this platform.

Ref: https://github.com/nodejs/node/pull/8864
PR-URL: https://github.com/nodejs/node/pull/8924


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Wyatt Preul <wpreul@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>