netbsd-tests: libc: fix strvis(3) overflow tests

These tests weren't run on x86 until CI grabbed them. It turns out,
there's a sign extension bug that surfaces on x86 with char being a
signed type.

NetBSD unearthed this when they took and improved the patch, so just
grab their solution until we get to merging in the latest version of
the test.

Reported by: CI (via ngie)
Fixes: 2f489a509e61 ("libc: fix some overflow scenarios in vis(3)")