Add fold_grind.t

Add fold_grind.t

This .t is designed to mostly replace reg_fold.t.  it provides much more
comprehensive tests, but has a huge number of failures that should be
TODOs that I found it difficult to classify, so have deferred adding it
until now, when in a few commits, those will be whittled way down.

And, those would only come in testing things that aren't character
classes, which are currently commented out, and reg_fold.t is relied on
for testing those.