Fix shortdoc examples

Fix shortdoc examples

Make sure that each example in shortdoc actually contains the function
it illustrates, and add a test for it.

* lisp/emacs-lisp/shortdoc.el (string, list, buffer, number):
Use the right functions in examples for string-version-lessp,
lax-plist-put, point-min and ffloor.
* test/lisp/emacs-lisp/shortdoc-tests.el: New test file.