Fix test data race in two FaultInjectionTest tests
Fix test data race in two FaultInjectionTest tests

Summary: Background sleeping tasks may conflict with test cleaning up. Wait for the sleeping tasks to finish before ending the test.

Test Plan: Run these tests.

Reviewers: andrewkr, yiwu

Reviewed By: yiwu

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61827

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