In Octave ≤ 5, it was impossible to take a function handle for a function in a
package (“+” folder). This bug has been fixed in Octave 6, so we can use the
same codepath as MATLAB in that case. See
https://savannah.gnu.org/bugs/index.php?46659
— use XLS instead of XLSX in testsuite datafiles
— use optimset instead of optimoptions
— use quadv/quadl/quadgk instead of integral
— fix race condition in load_m_file_data_legacy.m tests
— use fallback implementation for intersect(…, 'stable')