lib/locale.t: Fix broken test

The test that [:digit:] is a subset of [:xdigit:] failed in locales
where  [:digit:] matches 2 blocks of 10 digits, but the second block
isn't considered part of [:xdigit:].  This happens in Thai on Windows.
The POSIX standard http://pubs.opengroup.org/onlinepubs/9699919799/
does not use very clear language, but I'm taking it as meaning it is ok
for this to happen, so this revises the test to accept it.
PATCH: [perl #121340] lib/locale.t noisy+complaining but passes on Win32

It turns out that these messages were not printed as one would expect
under TAP, but were output using warn().
lib/locale.t: Update $variable name

As of commit b057411ddb1a3d8b6ab062d667c8e39f80cd7343, the meaning of
the variable is extended to beyond just being about 'folding', so change
the name to correspond.