[test] Update webconfig's tests accounting for cubicweb-file being now a package
WebconfigTC.test_locate_all_files and WebconfigTC.test_locate_resource
assumed that cubicweb-file was installed in <share>/cubes/file whereas,
from the latest release, it is now a standard Python package thus
installed in site-packages as cubicweb_file. We simply update base
directory of these two tests.
