test: fix fs.readFile('/dev/stdin') tests

The tests were creating the temp fixture file in both the parent
and the child process, leading to interesting race conditions on
the slower buildbots.

Rod notes that the tests started failing after putting the build
directory on a NFS mount.

Fixes: https://github.com/nodejs/io.js/issues/2261
PR-URL: https://github.com/nodejs/io.js/pull/2265


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>