os: ObjectStore::collect_metadata unit tests

And an emacs-executable reminder about how to run a specific test.

Signed-off-by: Loic Dachary <loic@dachary.org>
os: ObjectStore::create_backend unit tests

Signed-off-by: Loic Dachary <loic@dachary.org>
os: FileStore::create_backend unit tests

Assert that the fs_type is mapped to a backend that claims to have the
matching name.

Signed-off-by: Loic Dachary <loic@dachary.org>