test: mark fork regression test flaky on windows

See #3635 for details and failure
examples.

Ref: #3635
PR-URL: #4005
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/parallel.status

@@ -7,7 +7,8 @@ prefix parallel
 [true] # This section applies to all platforms
 
 [$system==win32]
-test-tls-ticket-cluster           : PASS,FLAKY
+test-tls-ticket-cluster              : PASS,FLAKY
+test-child-process-fork-regr-gh-2847 : PASS,FLAKY
 
 [$system==linux]
 test-http-client-timeout-event    : PASS,FLAKY