test: mark test-trace-events-fs-sync as flaky

test-trace-events-fs-sync has been failing ocasinally on FreeBSD. While
we don't have a fix, it should be marked as flaky.

Ref: https://github.com/nodejs/node/issues/21038

PR-URL: https://github.com/nodejs/node/pull/21039
Refs: https://github.com/nodejs/node/issues/21038


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Lance Ball <lball@redhat.com>