test: make sure O_NOATIME is present only in Linux

As it is, the test checks if the return value is `undefined` in other
platforms. But it should also make sure that the `O_NOATIME` should be
found only in Linux.

PR-URL: https://github.com/nodejs/node/pull/6614


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>