UMLTP2 — Predefined verdicts are not always reusable

UTP 2 provides the predefined verdict instances none, pass, inconclusive, fail, error. These instances are currenty represented by LiteralStrings of type 'verdict'. Using LiteralStrings is fine for the UTP test logging facility for TestCaseLog and TestSetLog simply refers to ValueSpecifications of type 'verdict'. But in situations where those predefined verdict instances shall be used in a different context e.g. in the context of types of Properties of a StructuredClassifier and InstanceSpecifications thereof, LiteralStrings cannot be really reused for Slot values. Therefore, the LiteralString should be reworked into InstanceSpecifications of type 'verdict' with their specification set to LiteralStrings (of type 'verdict') with name and value set to the 5 predefined verdicts.

Replace LiteralString verdict instances with InstanceSpecification verdict instances

As the issue description indicates the use of LiteralStrings as predefined verdict instances may prevent reuse of these predefined verdict instances in certain situations.
If one of the predefined verdict instances shall be used in a context other than associating test logs with these predefined verdicts, it happens that reuse is not possible because of composite associations between a UML metaclass and ValueSpecification.
For example, if there is a user-defined complex verdict, expressed as StructuredClassifer with Properties and a predefined verdict shall be assigned to one of the Properties in the context of an InstanceSpecification's Slot, it happens that Slot.value is a composite association to ValueSpecification. This means that the Slot physically owns the ValueSpecification, which is not possible then in the context of a predefined (i.e., read-only) model library.
Therefore, the proposal is to use InstanceSpecifications instead to express the predefined verdicts. By doing so, these verdict instances can be reused by means of InstanceValues.
The specification shall not, however, prevent using other ValueSpecifications as verdict instances in case of user-specific test models. It must be clearly stated that the use of InstanceSpecifications is simply the default UTP way to express verdict instances.