lntest: init SignerClient and test DeriveSharedKey

lntest/itest/lnd_test.go

@@ -14236,6 +14236,10 @@ var testsCases = []*testCase{
 		name: "node sign verify",
 		test: testNodeSignVerify,
 	},
+	{
+		name: "derive shared key",
+		test: testDeriveSharedKey,
+	},
 	{
 		name: "async payments benchmark",
 		test: testAsyncPayments,