itest+lntest: migrate lntemp to lntest

This commit performs the takeover that `lntemp` is now promoted to be
`lntest`, and the scaffolding is now removed as all the refactoring is
finished!

lntest/rpc/state.go

@@ -0,0 +1,19 @@
+package rpc
+
+import (
+	"github.com/lightningnetwork/lnd/lnrpc"
+)
+
+// =====================
+// StateClient related RPCs.
+// =====================
+
+// SubscribeState makes a rpc call to StateClient and asserts there's no error.
+func (h *HarnessRPC) SubscribeState() lnrpc.State_SubscribeStateClient {
+	client, err := h.State.SubscribeState(
+		h.runCtx, &lnrpc.SubscribeStateRequest{},
+	)
+	h.NoError(err, "SubscribeState")
+
+	return client
+}