TF-1822: Write test for all func of FileUtils class

(cherry picked from commit 562f151134416c674467e5a59919ea37adbb7e5f)

core/test/utils/file_utils_test.dart

@@ -0,0 +1,51 @@
+import 'package:core/utils/file_utils.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:path_provider_platform_interface/path_provider_platform_interface.dart';
+import 'package:plugin_platform_interface/plugin_platform_interface.dart';
+
+const String kApplicationDocumentsPath = 'applicationDocumentsPath';
+
+void main() {
+  const fileName = 'test_file';
+  const fileContent = 'Hello, World!';
+
+  group('fileUtils test', () {
+    setUp(() async {
+      PathProviderPlatform.instance = FakePathProviderPlatform();
+    });
+
+    test('Store private HTLM String to File', () async {
+
+      final file = await FileUtils().saveToFile(fileName, fileContent);
+
+      expect(await file.exists(), equals(true));
+
+      expect(await file.readAsString(), equals(fileContent));
+
+      file.delete();
+    });
+
+    test('Get HTML String from File Private', () async {
+
+      /// Create a temporary file that will be deleted after `getFromFile` is done
+      final file = await FileUtils().saveToFile(fileName, fileContent);
+
+      final htmlString = await FileUtils().getFromFile(fileName);
+
+      expect(htmlString.isNotEmpty, equals(true));
+
+      expect(htmlString, equals(fileContent));
+
+      file.delete();
+    });
+  });
+}
+
+class FakePathProviderPlatform extends Fake
+    with MockPlatformInterfaceMixin
+    implements PathProviderPlatform {
+  @override
+  Future<String?> getApplicationDocumentsPath() async {
+    return kApplicationDocumentsPath;
+  }
+}