Use addCleanup instead of tearDown in test_sql_persistence

TestCase.tearDown() is not called if setUp raises exception,
but we have to close backend and release lock before executing
other tests. The easiest way to do it is to use addCleanup
instead of tearDown for freeing resources.

Change-Id: I885bb7d491f4b3ee06a351f5d22087e23804e19b
Merge "Use addCleanup instead of tearDown in test_sql_persistence"