Create new testing helper file

This adds t/charset_tools.pl, and populates it with 2 functions removed
from t/test.pl.  The functions are changed very slightly to use the
variables $::IS_ASCII and $::IS_EBCDIC instead of recalculating this
information.

A new function byte_utf8a_to_utf8n() is also placed in charset_tools.
This takes the bytes that form a (ASCII-platform) UTF-8 string and
convert them to the bytes that form that string on the native platform,
hence just returns the input if run on an ASCII platform.