Refactor IntrospectionContextTestImpl in SuppressPropertiesBeanIntros…

[wx930910]

Fix BEANUTILS-551

Description

Replace test class IntrospectionContextTestImpl by mocking object and improve test design

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Motivation

• Decouple test class IntrospectionContextTestImpl from production interface IntrospectionContext.
• Extract attribute into test cases to make test condition more explict.
• Make test logic more clear by using method stub instead of method overriding.

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Key changed/added classes in this PR

• Created mocking object to replace test subclass IntrospectionContextTestImpl, decoupled test from production code.
• Extract removedProperties as a local variable to improve test logic and make test condition more explict.
• Make test logic more clear by using method stub instead of method overriding.
• Add Mockito dependency

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[garydgregory]

I'm going to say no here for the same reason as for the similar PRs proposed in other Commons components: this makes the code harder to follow and maintain and does not offer benefits.