Clarify what happens when mixing inheritance and default group redefinition [BVAL-711]

The TCK supposes that the order of the default group redefinition is respected whereas the spec is not that clear about that: it just says that the Default group of the child class contains the constraints of the redefined default group of the parent class.

Links:
* ML thread: http://lists.jboss.org/pipermail/beanvalidation-dev/2018-April/001491.html
* Spec paragraph: http://beanvalidation.org/2.0/spec/#constraintdeclarationvalidationprocess-groupsequence-formaldefinition

see https://hibernate.atlassian.net/browse/BVAL-711