I noticed the doc/development/testing_guide/best_practices.md still
referenced the `[run mysql]` tags, etc. They no longer work, so I
removed them, then realised I had better clean up the rest of
doc/development !

Also fixes instances in Markdown that broke the rule.

This reverts commit 676e0a32.
It was removed when we thought we were moving to Looker.

Related links:

* https://gitlab.com/gitlab-com/Product/issues/5
* https://gitlab.com/meltano/looker/issues/294