test: mark http-pipeline-flood flaky

test-http-pipeline-flood has been flaky on Windows for some time.
Hopefully, https://github.com/nodejs/node/pull/2862 fixes it and
lands soon, but until then, let's mark it as flaky.

PR-URL: https://github.com/nodejs/node/pull/3616
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

test/sequential/sequential.status

@@ -7,6 +7,7 @@ prefix sequential
 [true] # This section applies to all platforms
 
 [$system==win32]
+test-http-pipeline-flood                        : PASS,FLAKY
 
 [$system==linux]
 test-vm-syntax-error-stderr                     : PASS,FLAKY