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>