test: workaround test failure in memory_backend_check

Since the memory backend does not understand groups (yet), doing a
get_value for a key within an existing group will return the dummy group
data value ("BUXTON_GROUP_VALUE").

Work around the issue for now by validating that the call returns what
we expect it to.
direct: set layer UID for set_label

By not setting the layer UID before calling set_value here, the daemon
will open the user layer for the daemon itself. Instead, the daemon
should open the user layer for that user.

In a future commit, set_label will be disallowed for non-root clients,
but for consistency, make sure the UID is set.