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
Note: CI doesn't test on Catalina, yet (though I assume nodejs developers do). Test failures will indirectly block fixing of https://github.com/nodejs/node/issues/29216, which requires running notarization on Catalina, which requires tests passing on Catalina.
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines