test: add callback to fs.close() in test-fs-chmod

To avoid '[DEP0013] DeprecationWarning:
Calling an asynchronous function without callback is deprecated.'

PR-URL: https://github.com/nodejs/node/pull/12795


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>