test: refactor test-fs-assert-encoding-error

Check that callbacks are not executed when errors are expected to be
thrown.

PR-URL: https://github.com/nodejs/node/pull/13226


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>