test: mark test-util-callbackify flaky on AIX

Refs: #25068

PR-URL: #25284
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott

test/parallel/parallel.status

@@ -36,3 +36,5 @@ test-cli-node-options: PASS,FLAKY
 [$system==aix]
 # https://github.com/nodejs/node/issues/25029
 test-child-process-execfile: PASS,FLAKY
+# https://github.com/nodejs/node/issues/25068
+test-util-callbackify: PASS,FLAKY