test(server): exercise async client interop

crates/irtt-server/Cargo.toml

@@ -16,3 +16,6 @@ irtt-proto.workspace = true
 getrandom = "0.4.3"
 thiserror = "2.0.20"
 tokio = { version = "1.53.1", default-features = false, features = ["rt", "net", "time", "sync", "macros"] }
+
+[dev-dependencies]
+irtt-client = { workspace = true, features = ["tokio"] }