Use lexical-binding in delim-col.el and add tests

Use lexical-binding in delim-col.el and add tests

Thanks to Basil L. Contovounesios for additional cleanups.
For discussion, see the following thread:
https://lists.gnu.org/archive/html/emacs-devel/2019-05/msg00177.html

* lisp/delim-col.el: Use lexical-binding.

* test/lisp/delim-col-tests.el: New file.
(delim-col-tests-delimit-colummns-before-after)
(delim-col-tests-delimit-columns)
(delim-col-tests-delimit-columns-format/nil)
(delim-col-tests-delimit-columns-format/padding)
(delim-col-tests-delimit-columns-format/separator)
(delim-col-tests-delimit-columns-separator)
(delim-col-tests-delimit-columns-str-before-after)
(delim-col-tests-delimit-columns-str-separator)
(delim-col-tests-delimit-rectangle): New unit tests.