[tlbuild] Test failing on NetBSD

[Bruno Haible]

Karl Berry wrote:
>     is that in NetBSD the "C" locale apparently has ISO-8859-1 encoding,
> 
> I sincerely hope that isn't right.

Indeed, per POSIX
(<https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap07.html>
section "LC_CTYPE Category in the POSIX Locale"), the "C" locale must not
have ISO-8859-1 encoding. But it is valid for the "C" locale to have control
characters for all of 0x00..0x1F, 0x7F..0xFF.

Bruno