Continuous Integration (CI) at a high level is testing every code push from each developer, every time. While this sounds like an argues task, it does not have to be. Drupal and WordPress both have tools available that can help you test your code.
With Behat you can “walk” through your site one scenario at a time. You can fill out forms as a user would. Check for the existence of text on a page, even limit it to a pre-defined region of your site like the #footer. You can also log in a user with a specific role to test the admin or user function.
Of the three I am covering here, PHPUnit is the most involved. Each module or plugin you write needs a compatible test written. In this case, each class is proven to work with the test data provided. From the results, you can be ensured the new code has not broken the site. IMO the best way to work with PHPUnit is to use Test Driven Development (TDD); where you start with a failing test and write the code needed to get it to pass.
These three methods are only some of the tests you can run on your site but they cover the bulk of changes made by the development team. Testing your code as you go ensures a better product and allows you to act quickly if an issue arises; most times before it goes live.