Fix leak report by asan on DuplicateKeys test

Summary:
Deletes the transaction object at the end of the test.
Verified by:
- COMPILE_WITH_ASAN=1 make -j32 transaction_test
- ./transaction_test --gtest_filter="DBA**Duplicate*"
Closes https://github.com/facebook/rocksdb/pull/3470

Differential Revision: D6916473

Pulled By: maysamyabandeh

fbshipit-source-id: 8303df25408635d5d3ac2b25f309a3d15957c937

utilities/transactions/transaction_test.cc

@@ -5255,6 +5255,7 @@ TEST_P(TransactionTest, DuplicateKeys) {
     txn0->SetSavePoint();
     txn0->RollbackToSavePoint();
     ASSERT_OK(txn0->Commit());
+    delete txn0;
   }
 }