Fix failing tests (query variable integrality.)

[rschwarz]

Tests that were failing on Windows:

test_model.py::test_query_variable_attributes[HiGHS]
test_model.py::test_setting_variable_attributes[HiGHS]

For some reason, these did pass on Linux.

In the relevant functions, the return code of the C API function is not checked. If we check this, it also fails on Linux.

(It seems that the (internal) _integrality vector is not properly initialised by HiGHS at the point where try to read from it.)

[rschwarz]

First step: check return code everywhere!

[rschwarz]

After figuring out the reason for the errors, I added a simple workaround. So now everything passes.

I also reported the issue to HiGHS, but it's not really a bug: ERGO-Code/HiGHS#2261