fix: HTML self-closing tags parsing

description: self-closing tags in HTML blocks were closing prematurely
other tags than just themselves.

modified: omd_parser.ml
fix: do not parse as markdown contents of HTML blocks

description: between lines 3381 and 3385, there was a missing case, so some contents of HTML tags were always parsed as markdown
note: this commit also contains purely cosmetics modifications
modified: omd_parser.ml
fix: adapt test to new output for self-closing HTML tags (e.g., img)

modified: html_tags.html
fix: make test conform to newer line breaks policy

description: some non-significant newlines have been added to make the output prettier, this commit makes the test conform to the more recent policy
modified: dfb-reg-p-with-table.html