Replace ComparePair from kie-test-util with AssertJ assertions

We use a custom class ComparePair from kie-test-util artifact for comparing instances and their fields [1]. AssertJ provides assertions for that. See e.g. here [2].

[1] https://github.com/kiegroup/jbpm/blob/f70a465dd2621f5598f043631b8eb991142e05ba/jbpm-human-task/jbpm-human-task-core/src/test/java/org/jbpm/services/task/jaxb/AbstractTaskSerializationTest.java#L174
[2] http://joel-costigliola.github.io/assertj/assertj-core-features-highlight.html#field-by-field