write_callback_test: clean test directory before running tests
write_callback_test: clean test directory before running tests

Summary: write_callback_test fails if previous run didn't finish cleanly. Clean the DB before runing the test.

Test Plan: Run the test that see it doesn't fail any more.

Reviewers: andrewkr, yhchiang, yiwu, IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: kradhakrishnan, leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56859

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