tests: rpc_test: add test_unregister_handler
tests: rpc_test: add test_unregister_handler

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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