Convenience functions

Among other things, add `stack-core-filter-data' to create subfilters within elisp itself.

Also move test suite to its own directory and create a means to load that data quickly.
Yes, I was referring to the entire `stack-core-filter-data--item-in-tree` function.

I couldn't find a difference in the example you gave, but below is a point where they're different for me.  
Is this a relevant scenario? If it is (or if we're just not sure) we just leave things as is.

    (stack-core-filter-data--item-in-tree 1 '(2 (1 2 3) 1))
    ;; => (1 2 3)
    (oma 1 '(2 (1 2 3) 1))
    ;; => (1)

Where, of course, `oma` is defined as:

    (defun oma (item tree)
      (or (memq item tree)
          (assoc item tree)))