test: mark test-repl-user-error-handler as flaky

Signed-off-by: Aviv Keller <me@aviv.sh>
PR-URL: #64612
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Richard Lau <richard.lau@ibm.com>
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>

Author: avivkeller