test/objectstore: hold split_blob cache shards in unique_ptr

ExtentMap.split_blob, added in be93e121a98, creates the onode and buffer cache
shards as raw pointers and never frees them. once we build with ASan,
LeakSanitizer reports them (and the structures they own):

  Direct leak of 9928 byte(s) ... BlueStore::OnodeCacheShard::create
  Direct leak of 224 byte(s) ... BlueStore::BufferCacheShard::create
  SUMMARY: AddressSanitizer: 10288 byte(s) leaked in 8 allocation(s).

every other test in this file already wraps these shards in a unique_ptr,
declared before the collection that borrows them so they outlive it. do
the same here.

Signed-off-by: Kefu Chai <k.chai@proxmox.com>