fix flakey unit test
fix flakey unit test

Summary:
CancellationTest is flakey because of a race between server's task timeout and
client timeout. We address this be disabling server side task expirations and
always expecting client to timeout.

Reviewed By: yfeldblum

Differential Revision: D25191631

fbshipit-source-id: eddb0814c90063398ad9aa45e322866685e82096

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