Refactor SinkHandler in ReactorTest.java to improve test design

Description

I noticed that there is a test class SinkHandler implements production class BaseHandler to assist testing methd ReactorImpl:run(). This might not be the best priactice in unit testing and can be improved by leveraging mocking frameworks.

Current Implementation

•  SinkHandler implements BaseHandler and creates a new variable to keep tracking of the method invocation status for onDelivery(Devent).
• In test cases, after executing test target, the new variable will be used in assertion statement to check the execution status of onDelivery(Devent).

Proposed Implementation

• Replace SinkHandler with a mocking object created by Mockito.
• Extract the attribute into transfer(int, int) to surface counting logic.
• Use method stub to control the behavior of the mocking object.

Motivation

• Decouple test class SinkHandler from production class `BaseHandler`.
• Make test logic more clear by using method stub instead of method overriding.