Add smoke test for Shrine (#8)

Tests a simple journey: log in, buy a product, see the total in the cart.

This change also removes the empty unit test.

shrine/test/main_test.dart

@@ -2,10 +2,40 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'package:Shrine/app.dart';
+import 'package:Shrine/model/app_state_model.dart';
+import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:scoped_model/scoped_model.dart';
 
 void main() {
-  group('Unit tests', () {
-    test('This test always passes', () {});
+  testWidgets("smoke test", (tester) async {
+    AppStateModel model = AppStateModel();
+    model.loadProducts();
+
+    await tester.pumpWidget(ScopedModel<AppStateModel>(
+      model: model,
+      child: ShrineApp(),
+    ));
+
+    // Click through from the login screen.
+    await tester.tap(find.text("NEXT"));
+    await tester.pump();
+
+    // Ensure we populate the catalog page.
+    expect(find.text("Stella sunglasses"), findsOneWidget);
+    expect(find.text(r"$58"), findsOneWidget);
+
+    // Buy a product.
+    await tester.tap(find.text("Stella sunglasses"));
+    await tester.pump();
+
+    // Go to the shopping cart.
+    await tester.tap(find.byIcon(Icons.shopping_cart));
+    await tester.pumpAndSettle();
+
+    // Ensure that it appears, and that it computes total with tax and shipping.
+    expect(find.text("Subtotal:"), findsOneWidget);
+    expect(find.text(r"$68.48"), findsOneWidget);
   });
 }