I just published a blog post on writing tests for #Drupal 7: https://www.mediacurrent.com/blog/writing-simple-simpletest-tests-your-d7-module/
Another one on #Drupal8 tests: https://www.mediacurrent.com/blog/writing-simple-phpunit-tests-your-d8-module/
And the recording for today's #ContribHalfHour meeting stepping through writing a rudimentary test: https://www.youtube.com/watch?v=Sjz92q1oaQ8