org-list: Fix infloop when inserting an item

org-list: Fix infloop when inserting an item

* lisp/org-list.el (org-list-separating-blank-lines-number): When
  computing number of blank lines separating items, also count those
  in unparsed blocks, like example blocks.
* testing/lisp/test-org-list.el: Add tests.

In the following situation, with `org-blank-before-new-entry' set to
`auto' for `plain-list-item, a blank line should be inserted when
inserting the following item:

- item1
  #+BEGIN_EXAMPLE

  contents

  #+END_EXAMPLE