Fix flaky test with TestShadowRoute
Fix flaky test with TestShadowRoute

Summary: As above

Reviewed By: ftimyo

Differential Revision: D28296270

fbshipit-source-id: ca04a1b08b1776cdd992fbe9bd64681d67cdb304

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter