test: read() on dir on AIX does not return EISDIR

An upcoming change in libuv will remove the artificial EISDIR error.
Update the test to reflect that.

Refs: https://github.com/libuv/libuv/pull/2025

Backport-PR-URL: https://github.com/nodejs/node/pull/24103
PR-URL: https://github.com/nodejs/node/pull/23330


Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
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: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>