Summary:
In the folly/io/async tests, allow an extra 20ms of tolerance when performing
timeout checks on non-Linux platforms.

On Linux we can fairly reliably determine the amount of time that the test
spent waiting to be scheduled on the CPU.  This allows the timeout tests to be
relatively accurate even on overloaded systems.

On non-Linux platforms we cannot determine the time spent waiting to be
scheduled, and the `getSchedTimeWaiting()` implementation always returns 0.
This causes the tests to fail pretty frequently on Mac OS.  Adding an extra
20ms of tolerance should hopefully help reduce the test failure rate on Mac.

Reviewed By: yfeldblum

Differential Revision: D15464672

fbshipit-source-id: 2c3096df6a15dcdc68bf14c947736e4731929c0f