test: fix set-label check on Smack systems

The label "handle_label" is not necessarily to recognized label on a
Smack-enabled system, but the subsequent gets and sets to the group with
this label may fail.

The solution is to use a Smack builtin label, "*", instead, which
permits read and write access for all clients.

Note that this solution only makes sense for tests that are not
explicitly depending on Smack access control being enforced; tests in
check_smack.c and external Buxton consumers will generally not use the
"*" label for either groups or keys.
test: set BUXTON_ROOT_CHECK for relevant test suites

The check_buxton and check_daemon suites require BUXTON_ROOT_CHECK to be
set to "0" in order to bypass the UID 0 restriction for creating Buxton
groups.