Your first Drupal 8 module | Rob Varkonyi Drupal Consultant - https://robvarkonyi.com/blog/ultimate-guide-drupal-8/your-first-drupal-8-module

Automated Testing in Drupal 8 | Drupalize.Me - https://drupalize.me/series/automated-testing-drupal-8

Unit testing more complicated Drupal classes | Drupal 8 guide on Drupal.org - https://www.drupal.org/docs/8/phpunit/unit-testing-more-complicated-drupal-classes

Examples for Developers | Drupal.org (there are test examples here) - https://www.drupal.org/project/examples
PHPUnit in Drupal 8 | Drupal 8 guide on Drupal.org - https://www.drupal.org/docs/8/phpunit

A good example of a strict unit test in core - https://git.drupalcode.org/project/drupal/blob/HEAD/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php

PHPUnit Manual – Chapter 2. Writing Tests for PHPUnit - https://phpunit.de/manual/4.8/en/writing-tests-for-phpunit.html#writing-tests-for-phpunit.data-providers

PHPUnit Manual – Chapter 9. Test Doubles - https://phpunit.de/manual/6.5/en/test-doubles.html
phpspec/prophecy: Highly opinionated mocking framework for PHP 5.3+ - https://github.com/phpspec/prophecy

Writing a new Drupal 8 Module using Test Driven Development (TDD) | Oliver Davies - Full Stack Web Developer (Drupal, Symfony, Laravel, Linux) - https://www.oliverdavies.uk/articles/writing-drupal-module-test-driven-development-tdd/

TDD - Test Driven Drupal | Oliver Davies - Full Stack Web Developer (Drupal, Symfony, Laravel, Linux) - https://www.oliverdavies.uk/talks/tdd-test-driven-drupal/

Unit Testing Tutorial Part I: Introduction to PHPUnit | Juan Treminio - Dallas based senior web developer - https://jtreminio.com/blog/unit-testing-tutorial-part-i-introduction-to-phpunit/

Google Tech Talks - Misko Hevery - YouTube (Clean code and Unit testing) - https://www.youtube.com/playlist?list=PLD0011D00849E1B79

DevTernity 2017: Ian Cooper - TDD, Where Did It All Go Wrong - YouTube - https://www.youtube.com/watch?v=EZ05e7EMOLM&feature=youtu.be

Daniel Nitsche - How to unit test your code in Drupal 8 - DrupalSouth 2017 - Loft - https://www.youtube.com/watch?v=7FjjZ3OoD6Y

Writing Automated Tests in Drupal 8 - https://deninet.com/blog/2018/12/31/writing-automated-tests-drupal-8-part-1-test-types-and-set | https://deninet.com/blog/2019/01/13/writing-automated-tests-drupal-8-part-2-functional-tests | https://deninet.com/blog/2019/01/27/writing-automated-tests-drupal-8-part-3-unit-tests | https://deninet.com/blog/2019/02/10/writing-automated-tests-drupal-8-part-4-kernel-tests