rust: test AncestorsIterator without duplication...
rust: test AncestorsIterator without duplication
Up to now, it was acceptable to duplicate the test
in `test-rust-ancestor.py` that was relevant to `dagop`.
Now that we have more PyO3 implementations, it is better, and
actually more convincing, to express the tests in a common base
class (has technically to be a mixin to avoid running the tests
from the base class itself).