test: update cwd-enoent tests for AIX

On AIX you can not remove a directory that you are currently inside of
as it results in an EBUSY error. "EBUSY: resource busy or locked".
Updated the tests accordingly so that they are skipped on AIX.

PR-URL: https://github.com/nodejs/node/pull/2909


Reviewed-By: Ben Noordhuis <ben@strongloop.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>