runtests.sh: move start_flow to top-level
runtests.sh: move start_flow to top-level

Summary: I'll be reusing this in another code path in this script

Reviewed By: mroch

Differential Revision: D21900037

fbshipit-source-id: bc65bcda033953a4a456af17d8a5d6457afee417

mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow