add final check to integrity tests

[SierraGolf]

description
the aim is to let the user define a configuration on each integrity test that the method under test (equals, toString, hashCode) should be final or not. so that he can make sure that the intention of making an implementation non-overrideable or overrideable is covered by a test.

requirements

• the test should be optional
• the user should be able to enable the test by calling one method with a boolean parameter
• the boolean parameter should indicate whether the method under test must be final or not
• the method under test should be considered final if it is explicitly declared final or if its class is final

signature
Builder testExtensibility(boolean shouldBeExtendable)