[devtools] Re-implement generative tests using subtests
Generative tests as implemented in logilab.common.testib are not compatible
with tests runner other than lgc.pytest and this implementation differs from
the standard library, which has support for subtests_ since Python 3.4. Use
unittest2 to bridge the gap.
Maybe it'd be good to implement this on logilab.common.testlib side at some
point. Let's see how this gets received here first.
.. _subtests: https://docs.python.org/3/library/unittest.html#subtests