Refactor MockChannelListener with mocking object and improve testing logic

[wx930910]

Fix CAY-2717

Description

Refactor test class DataContextDataChannelEventsIT.java by using Mockito.

---

Motivation

• Decoupling test class MockChannelListener from production interface DataChannelListener
• Making test condition more clear by removing redundant overridden methods and new fields.
• Use Mockito.verify() to directly verify the behavior of the mocking object and make test condition more explict.

---

Key changed/added classes in this PR

• Creating mocking object to replace test subclass MockChannelListener, decoupled test from production code.
• Replacing assertation statement by using Mockito.verify() to verify the invocation status of graphChanged(GraphEvent), graphFlushed(GraphEvent) and graphRolledback(GraphEvent).