add new test for clone implementation

[SierraGolf]

aspects

• presence
  • is the method actually declared
• completeness
  • every property which is not excluded must be copied, meaning the original and the copy must have the same value (not reference) after copying
  • one can not assure 100% that the property is really copied or if by accident the default value of the object matches the values used for testing
• deep copy
  • every non-immutable and non-primitive property must be a deep copy, meaning the reference of the original and the copy must be different
  • can that actually be achieved with the current implementation of cg-proxy for interfaces, abstract and concrete classes?
  • may this be optional?
• null-safety
  • the implementation of the clone method must be able to handle null values for every property which is not excluded

notes

• does it make sense to compare the toString, equals and hashCode results of the cloned object to its original?