Add an individual test for request handler

test/unit/request/request_handler/request_handler_test.dart

@@ -0,0 +1,24 @@
+import 'package:dart_mediator/src/request/handler/request_handler.dart';
+import 'package:test/test.dart';
+
+import '../../../mocks.dart';
+
+void main() {
+  group('RequestHandler', () {
+    setUp(() {});
+
+    group('handle', () {
+      test('it handles the request', () {
+        var handled = false;
+        final mockRequest = MockRequest<int>();
+        final handler = RequestHandler<int, MockRequest<int>>.function((req) {
+          handled = true;
+          return 123;
+        });
+
+        expect(handler.handle(mockRequest), 123);
+        expect(handled, isTrue);
+      });
+    });
+  });
+}