main/attr: fix test/attr.test

musl outputs a different string for ENOTSUP

see https://savannah.nongnu.org/bugs/?62370

https://git.musl-libc.org/cgit/musl/tree/src/errno/__strerror.h#n85

uClibc uses the same string as glibc https://github.com/ffainelli/uClibc/blob/master/libc/string/_string_syserrmsgs.c#L109

That's why I think upstream is less likely to resolve the issue, it's something that Rich should fix, but I don't think he will