Edit comments.
Put all tests in single (module test racket ___).

Instead of multiple (module+ test ___).
Simplify $html-element-contents. Add to inline-tags list.
Revive redex testing.

- Only produce the $div portion of the grammar. Why? The markdown
  parser is only intended to handle block and inline elements that
  would go in a <body> element.

- The redex tester can generate xexprs like `(tag () "a" "b")` but the
  markdown parser always appends successive strings like `(tag ()
  "ab")` so do same to redex-generated ones for comparison. (A browser
  wouldn't care either way, so normalize them for test compare.)

- Fix typo from origianl: tag is acronym, singular.