Fix race condition in //folly/test:synchronized_test due to sharing RNG on multiple threads.
Fix race condition in //folly/test:synchronized_test due to sharing RNG on multiple threads.

Summary: Turned it into a thread_local instead. Only affects tests.

Reviewed By: yfeldblum

Differential Revision: D22744469

fbshipit-source-id: cbe8263d560f5abad27c7e61b1963aecf090a94e

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly