test/objectstore: fix memory leak test_bluestore_types.cc

test/objectstore: fix memory leak test_bluestore_types.cc

Previously, we had memory leak in the test_bluestore_types.cc tests where
`BufferCacheShard` and `OnodeCacheShard` objects were allocated with
raw pointers but never freed, causing leaks detected by AddressSanitizer.

ASan rightly pointed this out:

```
Direct leak of 224 byte(s) in 1 object(s) allocated from:
    #0 0x55a7432a079d in operator new(unsigned long) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_bluestore_types+0xf2e79d) (BuildId: c3bec647afa97df6bb147bc82eac937531fc6272)
    #1 0x55a743523340 in BlueStore::BufferCacheShard::create(BlueStore*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>>, ceph::common::PerfCounters*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/os/bluestore/Bl
ueStore.cc:1678:9
    #2 0x55a74330b617 in ExtentMap_seek_lextent_Test::TestBody() /home/jenkins-build/build/workspace/ceph-pull-requests/src/test/objectstore/test_bluestore_types.cc:1077:7
    #3 0x55a7434f2b2d in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.
cc:2653:10
    #4 0x55a7434b5775 in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:
2689:14
    #5 0x55a74347005d in testing::Test::Run() /home/jenkins-build/build/workspace/ceph-pull-requests/src/googletest/googletest/src/gtest.cc:2728:5
```
```
Direct leak of 9928 byte(s) in 1 object(s) allocated from:
    #0 0x7ff249d21a2d in operator new(unsigned long) /usr/src/debug/gcc/gcc/libsanitizer/asan/asan_new_delete.cpp:86
    #1 0x6048ed878b76 in BlueStore::OnodeCacheShard::create(ceph::common::CephContext*, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, ceph::common::PerfCounters*) /home/kefu/dev/ceph/src/os/bluestore/BlueStore.cc:1219
    #2 0x6048ed66d4f9 in GarbageCollector_BasicTest_Test::TestBody() /home/kefu/dev/ceph/src/test/objectstore/test_bluestore_types.cc:2662
    #3 0x6048ed820555 in void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2653
    #4 0x6048ed80c78a in void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*) /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2689
    #5 0x6048ed7b8bfa in testing::Test::Run() /home/kefu/dev/ceph/src/googletest/googletest/src/gtest.cc:2728
```

In this change, we replace raw pointer allocation with unique_ptr to
ensure automatic cleanup when the objects go out of scope.
`
Signed-off-by: Kefu Chai <tchaikov@gmail.com>