[test] Use stdlib doctest module instead of logilab-common class The former just works fine now but the later does not play well with py.test.

[test] Re-implement generative tests with subtests

[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