tests: rpc: adjust sstring serializer for current serializer protocol
tests: rpc: adjust sstring serializer for current serializer protocol

The sstring serializer was not in use, and so bit-rotted.

cloudius-systems/seastar,cloudius-systems/seastar,dreamsxin/seastar,sjperkins/seastar,kjniemi/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,kjniemi/seastar,sjperkins/seastar,kjniemi/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,sjperkins/seastar,dreamsxin/seastar,scylladb/seastar,scylladb/seastar,syuu1228/seastar,avikivity/seastar