Add links to resources

[opdavies]

• https://www.drupal.org/docs/8/testing
• https://www.drupal.org/docs/7/testing
• https://drupalize.me/series/automated-testing-drupal-8
• https://drupalize.me/series/testing-drupal-7-simpletest
• https://symfonycasts.com/screencast/phpunit
• https://course.testdrivenlaravel.com
• https://confidentlaravel.com
• https://phpunit.de
• http://butunclebob.com/ArticleS.UncleBob.TheThreeRulesOfTdd
• http://www.javiersaldana.com/articles/tech/refactoring-the-three-laws-of-tdd
• https://www.youtube.com/watch?v=AoIfc5NwRks (The Three Laws of TDD talk by Uncle Bob)
• https://thoughtbot.com/upcase/fundamentals-of-tdd