test: use '*' data labels where appropriate

The real tests for Smack label enforcement will live in check_smack.c,
so make sure the rest of the tests can always read and write data if
this would normally be prohibited due to the client's Smack label.

This is done by using the '*' (star) label, granting world read/write
access, unless the client lacks the required DAC permissions.

The one exception to this rule, for now, is the "default label" of
floor, which is assigned to a key when the client label is absent.
test: change System/User rules in test.load2

Since the tests may be built and run on Tizen 3.0, which uses a
three-domain model with Smack enabled, it is possible that 'make check'
will run with any of "_", "System", or "User" labels.

For upcoming tests to be included in check_smack.c, a few new Smack
rules need to be added to account for this three-domain model and to
work with buxton's notion of a default label (currently "_").

To make sure existing tests behave sanely when run under Tizen 3.0, make
sure "System" and "User" labels have mutual read/write access to the
default label.