[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).

Release notes
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.. toctree::
    :maxdepth: 1

    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14