add test for searching behavior

I added a test for the "search outside the default resource path" scenario pointed out by @hoverath in https://devhub.vr.rwth-aachen.de/VR-Group/Project_Phoenix/merge_requests/137#note_18001.
That said, this test should only be temporary, because by #447, the resource_root will be gone soon'ish. Then, every resource lookup will test the "search in path" behavior and thus the test becomes obsolete.