add test for OrderInstructions

test/protocol/models/order_instructions_test.dart

@@ -0,0 +1,52 @@
+import 'dart:convert';
+
+import 'package:tbdex/src/protocol/models/order_instructions.dart';
+import 'package:tbdex/tbdex.dart';
+import 'package:test/test.dart';
+import 'package:typeid/typeid.dart';
+
+import '../../helpers/test_data.dart';
+
+void main() async {
+  await TestData.initializeDids();
+
+  group('OrderInstructions', () {
+    test('can create a new order instruction', () {
+      final orderInstructions = OrderInstructions.create(
+        TestData.pfi,
+        TestData.alice,
+        TypeId.generate(MessageKind.rfq.name),
+        OrderInstructionsData(
+          payin: PaymentInstruction(instruction: 'just do it'),
+          payout: PaymentInstruction(instruction: 'just receive it'),
+        ),
+      );
+
+      expect(
+        orderInstructions.metadata.id,
+        startsWith(MessageKind.orderinstructions.name),
+      );
+      expect(
+        orderInstructions.metadata.kind,
+        equals(MessageKind.orderinstructions),
+      );
+      expect(orderInstructions.metadata.protocol, equals('1.0'));
+      expect(orderInstructions.data.payin.instruction, equals('just do it'));
+      expect(
+        orderInstructions.data.payout.instruction,
+        equals('just receive it'),
+      );
+    });
+
+    test('can parse and verify order instructions from a json string',
+        () async {
+      final orderInstructions = TestData.getOrderInstructions();
+      await orderInstructions.sign(TestData.pfiDid);
+      final json = jsonEncode(orderInstructions.toJson());
+      final parsed = await OrderInstructions.parse(json);
+
+      expect(parsed, isA<OrderInstructions>());
+      expect(parsed.toString(), equals(json));
+    });
+  });
+}