MacOS: iconv library behaves differently starting from MacOS 14 Somona - skip test "convert invalid UTF-8 to ISO-8859-1“

[Friedrich Beckmann]

Hi Ben,

pspp uses the native iconv library from Apple on MacOS. Apple has changed that 
library from MacOS 13 to MacOS 14. As a result the returned error codes seem 
different. That is the reason why the test "convert invalid UTF-8 to 
ISO-8859-1“ fails on MacOS 14. The problem is discussed here:

https://github.com/fredowski/homebrew-pspp/issues/5#issuecomment-1793464558

I made a patch to disable that test on MacOS such that the regression will work 
again on MacOS 14. I hope until Apple fixes the iconv library. 

Cheers

Fritz