* tests: add more asserts

* fix barvinok test

Includes changes equivalent to https://github.com/inducer/pycuda/pull/423



Co-authored-by: Antoine Martin <antoine@xpra.org>

Angus Gibson authored Jul 19, 2022 and Andreas Klöckner committed Jul 19, 2022

A recent change to pybind11 (1d81191, "Disable implicit
conversion of 0 to pybind11::handle.") removes implicit
conversions of "0" to a pybind handle, which the definition of
ISLError relied on. We should explicitly use a nullptr anyway,
which is compatible with newer and older pybind11 versions.