[devtools/testlib] Issue a RuntimeError when a generative tests is detected
Otherwise, generative tests *à la logilab-common* in CubicWebTC test cases
would silently not run (at least using logilab.common.pytest runner).
usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc