Merge: nitunit: Use markdown2

This PR makes nitunit use the new `markdown2` parser.

Changes:
* clean the warnings in `test_doc`
* migrate `nitunit` to `markdown2`
* fix some tests about indented Nit code blocks according to the CommonMark spec
* fix some tests about error location

This PR should close the bug signaled in #1394.

Pull-Request: #2725