test: fix test-fs-access when uid is 0

Superusers can open files with W_OK permission even though their
mode is set to 0444. This commit makes the test attempt to change
its uid to nobody on non-Windows platforms. Patch originally from
https://github.com/joyent/node/commit/28d0cbbd.

Fixes: https://github.com/iojs/io.js/issues/1031
PR-URL: https://github.com/iojs/io.js/pull/1037


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>