fix FutexTest
fix FutexTest

Summary: 2_ms seems too short. I am not sure whether it's feasible to check whether thread is waiting for an address. We could wait for longer to reduce false alarm.

Reviewed By: nbronson

Differential Revision: D5220819

fbshipit-source-id: 42f31206e9cb7f9addaa049d0e7cd995f6735f6c

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