Fix BackupEngineTest.ExcludeFiles memory leak (#11066)

Summary:
Valgrind was complaining about the test BackupEngineTest.ExcludeFiles. The cause is backup_engine not being freed similar to https://github.com/facebook/rocksdb/issues/9610.
```
==18228== Command: ./backup_engine_test --gtest_filter=BackupEngineTest.ExcludeFiles
==18228==
Note: Google Test filter = BackupEngineTest.ExcludeFiles
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from BackupEngineTest
[ RUN      ] BackupEngineTest.ExcludeFiles
[       OK ] BackupEngineTest.ExcludeFiles (16264 ms)
[----------] 1 test from BackupEngineTest (16273 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (16306 ms total)
[  PASSED  ] 1 test.
==18228==
==18228== HEAP SUMMARY:
==18228==     in use at exit: 14,099 bytes in 159 blocks
==18228==   total heap usage: 255,328 allocs, 255,169 frees, 497,538,546 bytes allocated
==18228==
==18228== 19 bytes in 1 blocks are possibly lost in loss record 4 of 67
==18228==    at 0x483BE63: operator new(unsigned long) (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==18228==    by 0x1E752D: void std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_construct<char*>(char*, char*, std::forward_iterator_tag) [clone .constprop.0] (basic_string.tcc:219)
==18228==    by 0x1F1898: _M_construct_aux<char*> (basic_string.h:251)
==18228==    by 0x1F1898: _M_construct<char*> (basic_string.h:270)
==18228==    by 0x1F1898: basic_string (basic_string.h:455)
==18228==    by 0x1F1898: construct<std::__cxx11::basic_string<char>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&> (new_allocator.h:146)
==18228==    by 0x1F1898: construct<std::__cxx11::basic_string<char>, const std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >&> (alloc_traits.h:483)
==18228==    by 0x1F1898: push_back (stl_vector.h:1189)
==18228==    by 0x1F1898: rocksdb::(anonymous namespace)::TestFs::NewWritableFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::FileOptions const&, std::unique_ptr<rocksdb::FSWritableFile, std::default_delete<rocksdb::FSWritableFile> >*, rocksdb::IODebugContext*) (backup_engine_test.cc:208)
==18228==    by 0x4B3583: rocksdb::NewWritableFile(rocksdb::FileSystem*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::unique_ptr<rocksdb::FSWritableFile, std::default_delete<rocksdb::FSWritableFile> >*, rocksdb::FileOptions const&) (read_write_util.cc:23)
==18228==    by 0x31C3A8: rocksdb::DBImpl::CreateWAL(unsigned long, unsigned long, unsigned long, rocksdb::log::Writer**) (db_impl_open.cc:1752)
==18228==    by 0x321A8C: rocksdb::DBImpl::Open(rocksdb::DBOptions const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, std::vector<rocksdb::ColumnFamilyDescriptor, std::allocator<rocksdb::ColumnFamilyDescriptor> > const&, std::vector<rocksdb::ColumnFamilyHandle*, std::allocator<rocksdb::ColumnFamilyHandle*> >*, rocksdb::DB**, bool, bool) (db_impl_open.cc:1852)
==18228==    by 0x322E7F: Open (db_impl_open.cc:1660)
==18228==    by 0x322E7F: rocksdb::DB::Open(rocksdb::Options const&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, rocksdb::DB**) (db_impl_open.cc:1637)
==18228==    by 0x1EE1CD: InitializeDBAndBackupEngine (backup_engine_test.cc:724)
==18228==    by 0x1EE1CD: rocksdb::(anonymous namespace)::BackupEngineTest::OpenDBAndBackupEngine(bool, bool, rocksdb::(anonymous namespace)::BackupEngineTest::ShareOption) (backup_engine_test.cc:732)
==18228==    by 0x217585: rocksdb::(anonymous namespace)::BackupEngineTest_ExcludeFiles_Test::TestBody() (backup_engine_test.cc:4232)
==18228==    by 0x296143: HandleSehExceptionsInMethodIfSupported<testing::Test, void> (gtest-all.cc:3899)
==18228==    by 0x296143: void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) (gtest-all.cc:3935)
==18228==    by 0x28A0A5: testing::Test::Run() [clone .part.0] (gtest-all.cc:3973)
==18228==    by 0x28A364: Run (gtest-all.cc:3965)
==18228==    by 0x28A364: testing::TestInfo::Run() [clone .part.0] (gtest-all.cc:4149)
...
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11066

Test Plan: make -j24 J=24 ROCKSDBTESTS_SUBSET=backup_engine_test valgrind_check_some

Reviewed By: ajkr

Differential Revision: D42297791

Pulled By: cbi42

fbshipit-source-id: db67982b27b91cc78e1a9f4a96da0cba7c9785b7

utilities/backup/backup_engine_test.cc

@@ -4389,6 +4389,8 @@ TEST_F(BackupEngineTest, ExcludeFiles) {
 
   // Includes check for leaked backups files
   AssertBackupInfoConsistency(/*allow excluded*/ false);
+
+  delete alt_backup_engine;
 }
 
 }  // namespace