test: fix some tests for Smack handling

In the four new bt-daemon tests, Smack rules must be cached, and client
Smack labels must be set manually.
test: fix labels for set_check bt-daemon test

The set_check test will break if Smack checks are enabled, so make sure
this test sets a key with a label for which the client has RW access.

In the following commit, Smack checks will be enforced for all tests
when the host OS is Smack-enabled.
Ignore access checks if Smack is not detected

In order to support OSes without Smack support enabled, disable Smack
access checks when the smackfs is not mounted at /sys/fs/smackfs.

Tests will still use the test/test.load2 file instead to ensure that we
have a consistent set of Smack rules in the test environment, not
dependent on the loaded rules from host OS.