test-suite warnings with Octave on shadowed built-in functions

[KrisThielemans]

Originally mentioned in https://github.com/jaeandersson/swig/pull/96#issuecomment-998434304

I get a ton of warnings like

Warning: function ./+li_math/atan.m shadows a built-in function

This is likely due to https://savannah.gnu.org/bugs/?46849. There doesn't seem to be anything we can do about this.