Fix UriRetriever test

Apart from extending and fixing the test, the return condition in the entry point of the Undefined constraint changed bc9438a5be53a01b5c17a8355ae79abd3919972d. That's probably the main point of this pull request.

In its more general former state the return condition has been hiding the fact that the test wasnt working and the validation of the retrieved schema was silently skipped. 

Couldn't identify any code that explicitly depends on the more general condition, such code would run into an exception.