test: Add test for evaluating future-based callbacks

test/notification_dispatcher_test.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:equatable/equatable.dart';
 import 'package:notification_dispatcher/src/notification_dispatcher.dart';
 import 'package:test/test.dart';
@@ -222,5 +224,29 @@ void main() {
       expect(callCount, 1);
       MockNotificationDispatcher.instance.clearAll();
     });
+
+    test('calls futures on post', () async {
+      var callCount = 0;
+      const delay = Duration(milliseconds: 100);
+
+      MockNotificationDispatcher.instance.addObserver(
+        instance,
+        name: observerName,
+        callback: (_) async => Future.delayed(
+          delay,
+          () => callCount++,
+        ),
+      );
+
+      unawaited(MockNotificationDispatcher.instance.post(name: observerName));
+      expect(callCount, 0);
+      await Future.delayed(delay);
+      expect(callCount, 1);
+
+      await MockNotificationDispatcher.instance.post(name: observerName);
+      expect(callCount, 2);
+
+      MockNotificationDispatcher.instance.clearAll();
+    });
   });
 }