Git clone the WordPress develop repository somewhere on your hard drive:

git clone git@github.com:WordPress/wordpress-develop.git

Open wordpress-develop/tests/phpunit/includes/phpunit6-compat.php in a text editor.

PHPUnit version 5: Comment out the class_alias() functions in phpunit6-compat.php because these break PhpStorm code completion. (These files aren’t actually used by the testing framework, we only downloaded them so they could be included in the Project Configuration’s Include Path.)

PHPUnit version 6 and up: Do the same thing as PHPUnit 5, the paragraph above, except leave this line uncommented:

class_alias( 'PHPUnitFrameworkTestCase', 'PHPUnit_Framework_TestCase' );

In PhpStorm, go to: Settings -> Languages & Frameworks -> PHP and add wordpress-develop/tests/phpunit/includes to your Include Path.

Use WP-CLI to generate the tests scaffolding.