Several methods for ensuring file (non-)existence in the ResourceTest
class are unnecessary. They are convenience methods to process multiple
resources which only have few consumers, which can iterate over the
resources on their own. Some of them even wrap two resources into an
array to pass to the method instead of calling the method two with
single resources. This change clean ups the according functionality.

Contributes to
https://github.com/eclipse-platform/eclipse.platform/issues/903