Tests needed for byte-based C locale

[Rich Felker <dalias@libc.org>]

Here are some basic tests I'd like to have for the byte-based C
locale, preferably most of them before committing the code and all
before a release containing it. Any help writing them (for the
libc-test framework) would be much appreciated. Short of that, even
just some quick sanity checks using existing programs (e.g. busybox
utils with regex/fnmatch usage) would be helpful.

Regex & fnmatch:

- Literals with arbitrary high bytes match.
- Brackets match byte values/ranges.

Multibyte functions (test r/non-r, string funcs, and btowc/wctob too):

- Successful round-trip for arbitrary bytes.
- Wchar values outside 0-7f & df80-dfff ranges give EILSEQ.

Iconv:

- Conversions to/from UTF-8 don't break in C locale.

Stdio:

- Encoding rule bound at time stream becomes oriented.
- All wide functions cause orientation/binding of encoding rule.
- Byte printf/scanf use current locale for %ls/%lc/%l[, not file's.
- Wide printf/scanf use current locale for %s/%c/%[, not file's.
- Wchar values outside 0-7f & df80-dfff ranges give EILSEQ.