Since CPython 3.8 .pyd files no longer look in PATH for loading libraries,
but require the DLL directory to be explicitely added via os.add_dll_directory().
This resulted in those tests failing with 3.8+ on Windows.
Add the DLL build directory with os.add_dll_directory() to fix them.
This was never noticed in CI because it only uses Python 3.7 and the
MSYS2 CPython still used the old behaviour until now.
Christoph Reiter