Reorder instance variables in backup test for proper destruction order
Reorder instance variables in backup test for proper destruction order

Summary:
As titled. This fixes the tsan error caused by logger_ being used in
backup_engine_'s destructor. It does not fix the transient unit test failure,
which is caused by MANIFEST file changing while backup is happening.

Test Plan:
verified the tsan error no longer happens on either success or
failure.

  $ COMPILE_WITH_TSAN=1 make -j32 backupable_db_test
  $ while ./backupable_db_test --gtest_filter=BackupableDBTest.CorruptionsTest ; do : ; done

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D54669

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