There is a problem with your test. It expects that both WicketRuntimeException is being thrown and that InternalErrorPage is rendered. Those are mutual exclusive.
The stack trace is:
ain@1, prio=5, in group 'main', status: 'RUNNING'
WicketRuntimeException is in wicket-core.jar. In the stack trace the only class from -core is BaseWicketTester. Url and UrlDecoder are in wicket-request which do not see WicketRuntimeException.
Even re-working BaseWicketTester to handle this it will be a change in the behavior which will affect many applications. Currently they may expect that an exception is being thrown in some erroneous case but with this change there wont be an exception anymore but normal return with lastRenderedPage == InternalErrorPage.
I agree that InternalErrorPage is more correct but it is a bit late to change this behavior.