test: mark cluster-net-send test flaky on windows

See https://github.com/nodejs/node/issues/3957 for details and examples
failures.

Ref: https://github.com/nodejs/node/issues/3957
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/node/pull/4006

test/parallel/parallel.status

@@ -7,6 +7,7 @@ prefix parallel
 [true] # This section applies to all platforms
 
 [$system==win32]
+test-cluster-net-send                : PASS,FLAKY
 test-tls-ticket-cluster              : PASS,FLAKY
 test-child-process-fork-regr-gh-2847 : PASS,FLAKY