add symmetry check to equals integrity test

[SierraGolf]

background
the transitivity requirement on the equals implementation can only be tested partially by testing the reflexive requirement of the class under test. in order to allow a symmetry check for multiple related classes there should be an optional test which checks two or more given classes to be symmetric to the class under test.

requirements

• the interface should allow the user to provide more than one class
• class can but do not have to be related (sharing an interface or being part of the same class hierarchy)

signature
Builder checkSymmetryTo( Class... otherClasses)