Xtend 2.29.0 treats JUnit imports as access restricted errors

[JanWittler]

As of Xtend 2.29.0, any JUnit import fails to be imported in an Xtend file with the warning Access restriction: The type <JUnit annotator type> is not accessible due to restriction on required project tools.vitruv.<project>. As such, currently the workaround of reducing the severity of such an error to warning is required. We should investigate why the import is marked as access restricted.