test: fix test-fs-access.js

On non-windows supported platforms, fs.access(readOnlyFile, W_OK, ...)
is expected to fail, but always succeeds if node runs as the super user,
which is often the case for tests running on our continuous integration
platform.

This change makes the test try to change its process user id to nobody
on non-windows platforms so that the above mentioned test can pass and
still perform the actual desired test. If changing the process user id
to a nobody is not possible, then the test checks that
fs.access(readOnlyFile, W_OK, ...) actually succeeds.

Fixes #9033.

Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy J Fontaine <tjfontaine@gmail.com>