[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).
[devtools] stop using lgc.testlib.TestCase
It's become incompatible with unittest(2).TestCase due to generative
tests. Just keep the datadir/datapath and assertCountEqual bits in our
BaseTestCase.