client-classes should allow definition of a class with no test line.

See: https://kea.readthedocs.io/en/kea-2.4.1/arm/dhcp4-srv.html#reserving-client-classes-in-dhcpv4

In global client-classes as you should be able to define an client class that doesn't have a test. Then select them in the reservation. Should still allow selection of client-classes that have a test inside the reservation as well, so no need to change anything inside reservations mechanism.

Won't need patch notes as is an oversight contained to the 0.3 version.