- On some versions of FreeBSD, setting the "UTF-8" locale
  succeeds, but a subsequent "nl_langinfo(CODESET)" fails
- adding a check for this in the coercion logic means that
  coercion will happen on systems where this check succeeds,
  and will be skipped otherwise
- that way CPython should automatically adapt to changes in
  platform behaviour, rather than needing a new release to
  enable coercion at build time
- this also allows UTF-8 to be re-enabled as a coercion
  target, restoring the locale coercion behaviour on Mac OS X

The problem with dynamically adaptive tests is that they
can sometimes pass without actually testing anything useful.

On Linux and Mac OS X, if setlocale works, we also expect
nl_langinfo(CODESET) to *always* work for the coercion target
locales. The tests now reflect this by always assuming the
target locale will work if setlocale succeeds when running
on Linux or Mac OS X.