test: avoid running fsync on directory on AIX

On AIX the underlying fsync system call returns EBADF on a file
descriptor for an open directory. So avoid running fsync on it.

PR-URL: https://github.com/nodejs/node/pull/21298


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>