test: mark empty udp tests flaky on OS X

They fail on OS X 10.15 (aka "Catalina"), but pass on earlier OS X.

Refs: https://github.com/nodejs/node/issues/30030
Refs: https://github.com/nodejs/build/pull/2189#issuecomment-589767606

PR-URL: https://github.com/nodejs/node/pull/31936


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>