Fix DBTest2.BackupFileTemperature memory leak (#9610)

Summary:
Valgrind was failing with the below error because we forgot to destroy
the `BackupEngine` object:

```
==421173== Command: ./db_test2 --gtest_filter=DBTest2.BackupFileTemperature
==421173==
Note: Google Test filter = DBTest2.BackupFileTemperature
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from DBTest2
[ RUN      ] DBTest2.BackupFileTemperature
--421173-- WARNING: unhandled amd64-linux syscall: 425
--421173-- You may be able to write your own handler.
--421173-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
--421173-- Nevertheless we consider this a bug.  Please report
--421173-- it at http://valgrind.org/support/bug_reports.html.
[       OK ] DBTest2.BackupFileTemperature (3366 ms)
[----------] 1 test from DBTest2 (3371 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (3413 ms total)
[  PASSED  ] 1 test.
==421173==
==421173== HEAP SUMMARY:
==421173==     in use at exit: 13,042 bytes in 195 blocks
==421173==   total heap usage: 26,022 allocs, 25,827 frees, 27,555,265 bytes allocated
==421173==
==421173== 8 bytes in 1 blocks are possibly lost in loss record 6 of 167
==421173==    at 0x4838DBF: operator new(unsigned long) (vg_replace_malloc.c:344)
==421173==    by 0x8D4606: allocate (new_allocator.h:114)
==421173==    by 0x8D4606: allocate (alloc_traits.h:445)
==421173==    by 0x8D4606: _M_allocate (stl_vector.h:343)
==421173==    by 0x8D4606: reserve (vector.tcc:78)
==421173==    by 0x8D4606: rocksdb::BackupEngineImpl::Initialize() (backupable_db.cc:1174)
==421173==    by 0x8D5473: Initialize (backupable_db.cc:918)
==421173==    by 0x8D5473: rocksdb::BackupEngine::Open(rocksdb::BackupEngineOptions const&, rocksdb::Env*, rocksdb::BackupEngine**) (backupable_db.cc:937)
==421173==    by 0x50AC8F: Open (backup_engine.h:585)
==421173==    by 0x50AC8F: rocksdb::DBTest2_BackupFileTemperature_Test::TestBody() (db_test2.cc:6996)
...
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9610

Test Plan:
```
$ make -j24 ROCKSDBTESTS_SUBSET=db_test2 valgrind_check_some
```

Reviewed By: akankshamahajan15

Differential Revision: D34371210

Pulled By: ajkr

fbshipit-source-id: 68154fcb0c51b28222efa23fa4ee02df8d925a18

db/db_test2.cc

@@ -6990,13 +6990,17 @@ TEST_F(DBTest2, BackupFileTemperature) {
   for (auto info : infos) {
     temperatures.emplace(info.file_number, info.temperature);
   }
-  BackupEngine* backup_engine;
+
+  std::unique_ptr<BackupEngine> backup_engine;
+  {
+    BackupEngine* backup_engine_raw_ptr;
     auto backup_options = BackupEngineOptions(
         dbname_ + kFilePathSeparator + "tempbk", backup_env.get());
-  auto s = BackupEngine::Open(backup_env.get(), backup_options, &backup_engine);
-  ASSERT_OK(s);
-  s = backup_engine->CreateNewBackup(db_);
-  ASSERT_OK(s);
+    ASSERT_OK(BackupEngine::Open(backup_env.get(), backup_options,
+                                 &backup_engine_raw_ptr));
+    backup_engine.reset(backup_engine_raw_ptr);
+  }
+  ASSERT_OK(backup_engine->CreateNewBackup(db_));
 
   // checking src file src_temperature hints: 2 sst files: 1 sst is kWarm,
   // another is kUnknown