Fix tests for ExecutorLoopController timer.
Fix tests for ExecutorLoopController timer.

Summary:
Using a ThreadPoolExecutor instead of calling getCPUExecutor() to get one.
getCPUExecutor() by default returns an inline executor, which doesn't ensure that sleep is triggered on the same executor as the fiber loop.

Reviewed By: andriigrynenko

Differential Revision: D18353758

fbshipit-source-id: 6810d11b12de3f62ac5286dba00d2b4a9d447eba

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