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: libuv/libuv#2025

PR-URL: #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>

Author: bnoordhuis

test/parallel/test-fs-readfile-error.js

@@ -25,9 +25,9 @@ const fs = require('fs');
 
 // Test that fs.readFile fails correctly on a non-existent file.
 
-// `fs.readFile('/')` does not fail on FreeBSD, because you can open and read
-// the directory there.
-if (common.isFreeBSD)
+// `fs.readFile('/')` does not fail on AIX and FreeBSD because you can open
+// and read the directory there.
+if (common.isAIX || common.isFreeBSD)
   common.skip('platform not supported.');
 
 const assert = require('assert');