Add mockito extension for mocking final methods.

Today we were getting NullPointerExceptions when mocking the final
methods of a class. We learned that by default mockito does not support
mocking final methods. In ordre to enable it, mockito extension should
be added to the test resources.

References:

* https://github.com/mockito/mockito/wiki/What%27s-new-in-Mockito-2#mock-the-unmockable-opt-in-mocking-of-final-classesmethods

src/test/resources/mockito-extensions/org.mockito.plugins.MockMaker

@@ -0,0 +1 @@
+mock-maker-inline