Fix timing-out AtomicHashMapTests
Fix timing-out AtomicHashMapTests

Summary: Under load, this test takes a ludicrously long time. Instead of iteration-based end conditions, have a time-based end condition. This is appropriate for this test's purpose: finding race conditions.

Reviewed By: yfeldblum

Differential Revision: D40251567

fbshipit-source-id: 45b4a92ab3dedcb719e620930be1af692a5f14cd

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