Add test for image.loading_builder.0.dart (#158248)

Contributes to https://github.com/flutter/flutter/issues/130459

It adds a test for
- `examples/api/lib/widgets/image/image.loading_builder.0.dart`

examples/api/test/widgets/image/image.loading_builder.0_test.dart

@@ -0,0 +1,65 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_api_samples/widgets/image/image.loading_builder.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  // The app being tested loads images via HTTP which the test
+  // framework defeats by default.
+  setUpAll(() {
+    HttpOverrides.global = null;
+  });
+
+  testWidgets('The loading builder returns the child when there is no loading progress', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.LoadingBuilderExampleApp(),
+    );
+    await tester.pumpAndSettle();
+
+    final Image image = tester.widget<Image>(find.byType(Image));
+    final ImageLoadingBuilder loadingBuilder = image.loadingBuilder!;
+    final BuildContext context = tester.element(find.byType(Image));
+
+    const SizedBox child = SizedBox(key: Key('child'));
+
+    await tester.pumpWidget(loadingBuilder(context, child, null));
+
+    expect(find.byWidget(child), findsOne);
+    expect(find.byType(CircularProgressIndicator), findsNothing);
+  });
+
+  testWidgets('The loading builder returns a circular progress indicator when loading', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      const example.LoadingBuilderExampleApp(),
+    );
+    await tester.pumpAndSettle();
+
+    final Image image = tester.widget<Image>(find.byType(Image));
+    final ImageLoadingBuilder loadingBuilder = image.loadingBuilder!;
+    final BuildContext context = tester.element(find.byType(Image));
+
+    const SizedBox child = SizedBox(key: Key('child'));
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: loadingBuilder(
+          context,
+          child,
+          const ImageChunkEvent(
+            cumulativeBytesLoaded: 1,
+            expectedTotalBytes: 10,
+          ),
+        ),
+      ),
+    );
+
+    expect(find.byWidget(child), findsNothing);
+    expect(find.byType(CircularProgressIndicator), findsOne);
+  });
+}