Add tests for material_state_border_side.0_test.dart (flutter#151089)

Contributes to flutter#130459

It adds a test for
- `examples/api/test/material/material_state/material_state_border_side.0_test.dart`

Author: ValentinVignal

dev/bots/check_code_samples.dart

@@ -311,7 +311,6 @@ class SampleChecker {
 final Set<String> _knownMissingTests = <String>{
   'examples/api/test/material/bottom_app_bar/bottom_app_bar.2_test.dart',
   'examples/api/test/material/bottom_app_bar/bottom_app_bar.1_test.dart',
-  'examples/api/test/material/material_state/material_state_border_side.0_test.dart',
   'examples/api/test/material/material_state/material_state_outlined_border.0_test.dart',
   'examples/api/test/material/material_state/material_state_property.0_test.dart',
   'examples/api/test/material/selectable_region/selectable_region.0_test.dart',

examples/api/test/material/material_state/material_state_border_side.0_test.dart

@@ -0,0 +1,56 @@
+// 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 'package:flutter/material.dart';
+import 'package:flutter_api_samples/material/material_state/material_state_border_side.0.dart'
+    as example;
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  Finder findBorderColor(Color color) {
+    return find.byWidgetPredicate((Widget widget) {
+      if (widget is! Material) {
+        return false;
+      }
+      final ShapeBorder? shape = widget.shape;
+      if (shape is! OutlinedBorder) {
+        return false;
+      }
+      return shape.side.color == color;
+    });
+  }
+
+  testWidgets(
+    'FilterChip displays the correct border when selected',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            body: example.MaterialStateBorderSideExampleApp(),
+          ),
+        ),
+      );
+
+      expect(findBorderColor(Colors.red), findsOne);
+    },
+  );
+
+  testWidgets(
+    'FilterChip displays the correct border when not selected',
+    (WidgetTester tester) async {
+      await tester.pumpWidget(
+        const MaterialApp(
+          home: Scaffold(
+            body: example.MaterialStateBorderSideExampleApp(),
+          ),
+        ),
+      );
+
+      await tester.tap(find.byType(FilterChip));
+      await tester.pumpAndSettle();
+
+      expect(findBorderColor(const Color(0xff79747e)), findsOne);
+    },
+  );
+}