The header `mbed.h` pulls in headers from a number of Mbed OS
components that are not necessarily used. It was intended for user
applications only, and libraries and unit tests should explicitly
include library headers it uses to limit dependencies.
This change avoids having to link unnecessary libraries in storage
unit tests' CMake definitions.
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/.