JXPATH-198: Refactor Context to improve test logic

[wx930910]

JIRA

Fixes XPATH-198

Description

Refactor test class Context

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Motivation

• Decouple test class Context from production interface ExpressionContext.
• Remove the redundant test child class Context
• Remove the redundant overridden methods that return default values.
• Make testing logic more explict.

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Key changed/added classes in this PR

• Created mocking object to replace test subclass Context, decoupled test from production code.
• Created a method that return the mocking object for reuse purpose.
• Use method stub to replace method overriding to control mocking object's behavior.
• Remove redundant overridden methods to improve test logic.
• Add Mockito dependency.

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[garydgregory]

Exact same comment as apache/commons-codec#94 (comment)