test: skip spawn_setuid_setgid if we get EACCES

More often than not, users run the test suite from /root, which means
the 'nobody' user won't be able to access the test runner and the test
will fail because we drop privileges to that user. Just skip it then.

PR-URL: https://github.com/libuv/libuv/pull/66
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>