MDEV-32249 strings/ctype-ucs2.c:2336: my_vsnprintf_utf32: Assertion `(n % 4) == 0' failed in my_vsnprintf_utf32 on INSERT

The crash inside my_vsnprintf_utf32() happened correctly,
because the caller methods:
  Field_string::sql_rpl_type()
  Field_varstring::sql_rpl_type()
mis-used the charset library and sent pure ASCII data to the
virtual function snprintf() of a utf32 CHARSET_INFO.

It was wrong to use Field::charset() in sql_rpl_type().
We're printing the metadata (the data type) here, not the column data.
The string contraining the data type of a CHAR/VARCHAR column
is a pure ASCII string.

Fixing to use res->charset() to print, like all virtual implementations
of sql_type() do.

Review was done by Andrei Elkin.
Thanks to Andrei for proposing MTR test improvents.