UTP12 — Base class of TestComponents must be instances of BehavioredClassifier, too

Since TestComponents can define test-relevant behavior, it must be ensured by definition that each base class of a TestComponent is also a BehavioredClassifier.

Behaviors for test cases can be expressed as operations in the test component, for example. To make this formally and by definition possible (and if we do not change the base class of TestComponent to Property - see previously submitted issue by me), we should also extend BehavioredClassifier and make clear that a base class, to which <<TestComponent>> is applied must be both a StructuredClassifier and BehavioredClassifier.

RTF agreed on narrowing the possible metaclasses the stereotype <<TestComponent>> down to uml::Class. Furthermore, uml::Behavior and uml::AssociationClass is excluded from the list of extendable metaclasses by a constraint, because they just do not make sense to be treated as a test component. This results in three concrete metaclasses where <<TestComponent>> can be applied to, which are: Class, Component and Node.