Marek,
To the question "why not create a real Entity instance?"
In this case, i would have been copy-pasting the same code from my
source code into the test code in order to create a real entity
instance here in the unit test. That is something I do not want to do;
instead, I want to do some verifying and assertions in the "stubbed
entity" to check my source code, rather than copy/paste/duplication my
source code here in the test, just re-create real Entity instance.