ISPN-3176 Add tests for compatibility mode to server

These tests do not use Memcached for writing, only for reading. If we wanted to test also write ops for Memcached, we would need to specify a custom marshaller for the compatibility mode and this would mean installing a jar file with that class into the modules (is there any other option than installing it as a new module?). I can do that if desired but IMO the current tests might be sufficient.

Martin