test: mark test-child-process-execsync flaky on AIX

Refs: #24921

PR-URL: #25031
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

Author: Trott

test/sequential/sequential.status

@@ -26,5 +26,7 @@ test-http2-large-file: PASS, FLAKY
 [$system==freebsd]
 
 [$system==aix]
+# https://github.com/nodejs/node/issues/24921
+test-child-process-execsync: PASS, FLAKY
 
 [$arch==arm]