User unsubscribed on Q&A topic shouldn’t get email notification when user leave comment on other answer

Pre-condition:
1. At least two users should be created and logged
2. Both of users should have VIEW_TOPICS and CREATE_POST permissions
3. At least one branch created
4. A branch has Q&A topic with some answers by User1
5. User2 should be subscribed on Q&A topic and unsubscribe on branch
6. User1 is author of Q&A topic