test: mark fork regression test flaky on windows

See https://github.com/nodejs/node/issues/3635 for details and failure
examples.

Ref: https://github.com/nodejs/node/issues/3635
PR-URL: https://github.com/nodejs/node/pull/4005
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/parallel.status

@@ -8,6 +8,7 @@ prefix parallel
 
 [$system==win32]
 test-tls-ticket-cluster              : PASS,FLAKY
+test-child-process-fork-regr-gh-2847 : PASS,FLAKY
 
 [$system==linux]
 test-http-client-timeout-event    : PASS,FLAKY