bug#13568: 24.3.50; wishlist: add a `group-p' predicate

[Drew Adams]

> > Subject line says it all: Please add a predicate `group-p' that
> > returns non-nil if its symbol arg is a loaded customize group
> > (i.e. defined by a defgroup that has been evaluated).
> 
> What the use case for such a function?

Uh, to test whether a given symbol corresponds to "a loaded
customize group (i.e. defined by a defgroup that has been
evaluated)."

Clearly (see the rest of the bug report), it is not clear
how Lisp code can test for this.  Property `custom-group'
is not the answer.  Nor, apparently, is a combination of
properties `custom-group' and `custom-autoload'.  How to
test for this condition?