watchman: fix racy subscription test
watchman: fix racy subscription test

Summary:
provide an appropriate `accept` lambda that tests for
our assertion so that the `waitForSub` routine waits for our
condition to be true as intended.

Reviewed By: chadaustin

Differential Revision: D20587611

fbshipit-source-id: e1681c0fdf8703724aea43c044d5d830945edbd3

nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman