Move storage stubs from UNITTESTS/stubs into components inside the
top-level storage directory. Specifically,
* storage/blockdevice/tests/UNITTESTS/doubles/ for BlockDevice stubs.
* storage/kvstore/filesystemstore/tests/UNITTESTS/ for a stub used by
the FileSystemStore test. The stub has been renamed from
kv_config_stub.cpp to filesystemstore_kv_config_stub.cpp, to make it
evident why it doesn't go into storage/kvstore/kv_config/.
Lingkai Dong