[devtools] "fix" boundary condition for autofill maxvalue in an IntervalBoundConstraint is inclusive. If the min and max values happened to coincide, we would call random.choice on an empty list, which it didn't like. This is still broken in the case of BoundaryConstraint('>', minvalue), as we assume the minvalue is in the range, but that's something for another time.

[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