test: mark test-child-process-fork-dgram as flaky

PR-URL: #8274
Reviewed-By: joaocgreis - João Reis <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>

Author: mhdawson

test/parallel/parallel.status

@@ -34,3 +34,6 @@ test-regress-GH-1899                 : FAIL, PASS
 # localIPv6Hosts list from test/common.js.
 test-https-connect-address-family : PASS,FLAKY
 test-tls-connect-address-family : PASS,FLAKY
+
+#covered by https://github.com/nodejs/node/issues/8271
+test-child-process-fork-dgram            : PASS, FLAKY