direct: improve Smack checks for set_value

The group check is still missing, so I added a FIXME there, and left a
dummy Smack check in its place for now.

This should restore the checking logic to the previous state, adapted to
the new API.
test: fix tests that were broken on Smack-enabled systems

For daemon tests that do not use check_bt_daemon, the client Smack
labels need to be initialized.