Fix tested-date field in YAML for several lessons

[charlottejmc]

It was recently noticed that several lessons don't display the 'tested date' field, because it was written tested-date in the YAML rather than tested_date (underscore required). I am making this small change in:

• en/lessons/creating-an-omeka-exhibit
• en/lessons/sustainable-authorship-in-plain-text-using-pandoc-and-markdown
  • and pt/licoes/autoria-sustentavel-texto-simples-pandoc-markdown
• en/lessons/up-and-running-with-omeka