The ExtentMap.reshard_failure test was leaking memory by not properly cleaning up the OnodeCacheShard and BufferCacheShard objects it created.

ASan reported:
  Direct leak of 9928 byte(s) in 1 object(s) allocated from:
    #1 BlueStore::OnodeCacheShard::create() BlueStore.cc:1221
    #2 ExtentMap_reshard_failure_Test::TestBody() test_bluestore_types.cc:1244

  Direct leak of 224 byte(s) in 1 object(s) allocated from:
    #1 BlueStore::BufferCacheShard::create() BlueStore.cc:1680
    #2 ExtentMap_reshard_failure_Test::TestBody() test_bluestore_types.cc:1246

  SUMMARY: AddressSanitizer: 10288 byte(s) leaked in 8 allocation(s).

Fix by:

1. Wrapping coll and onode in an additional scope block to ensure they are destroyed before the cache shards (releasing all blob references)
2. Adding proper cleanup with delete bc and delete oc at test end

This matches the cleanup pattern used in BlueStoreFixture::TearDown().

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