Improve GeneratorShortCircuits test relevancy

[sebalaig]

• Do you want to request a feature or report a bug?
  Potential bug in serialization/deserialization of GeneratorShortCircuitExtension.

• What is the current behavior?
  As notified Add xml serializers for shortcircuit extensions (#389) (requires !436) #437 (comment), the road trip test of GeneratorShortCircuit extension may be improved because every member of the extension have the same value in generatorShort...

This is also the case in the powsybl-core test suite => to be fixed also.

• If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem

• What is the expected behavior?
  It would be better if the extension attributes would have different values.

• What is the motivation / use case for changing the behavior?
  Avoid unseen serialization/deserialization error.

• Please tell us about your environment:

  • PowSyBl Version: ...
  • OS Version: ...

• Other information (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow, spectrum, etc)

(if a question doesn't apply, you can delete it)