reg_fold.t: Test bracketed character classes
These were removed when things were very broken, but now they work,
except for things like
"\N{LATIN SMALL LIGATURE FFI}" =~ /[a-z]{3}/i
where the multi-char fold crosses single bracketed character class
boundaries. These will probably never be fixed in Perl in the general
case (using \F and fc() instead), but I expect that
"\N{LATIN SMALL LIGATURE FFI}" =~ /[f][f][i]/i
will eventually be changed so the brackets are optimized away, and will
work. Then these TODOs will start passing.