tests: add a test for guest authentication

tests: add a test for guest authentication

This verifies that smbd always adds BUILTIN\Guests to the guest token which is
required for guest authentication.

Currently the guest token depends on the on-disk configured group mappings. If
there's an existing group mapping for BUILTIN\Guests, but LOCALSAM\Guest is not
a member, the final guest token won't contain BUILTIN\Guests.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13944

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>