reg_fold.t: Test bracketed character classes

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.