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: nodejs#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>

test/parallel/test-child-process-uid-gid.js

@@ -3,6 +3,11 @@ const common = require('../common');
 const assert = require('assert');
 const spawn = require('child_process').spawn;
 
+if (process.getuid() === 0) {
+  common.skip('as this test should not be run as `root`');
+  return;
+}
+
 const expectedError = common.isWindows ? /\bENOTSUP\b/ : /\bEPERM\b/;
 
 assert.throws(() => {