Basic kernel test fails with Type of Drupal\KernelTests\Core\Entity\EntityKernelTestBase::$modules must be array
From KernelTestBase:
 /**
  * Modules to enable.
  *
  * The test runner will merge the $modules lists from this class, the class
  * it extends, and so on up the class hierarchy. It is not necessary to
  * include modules in your list that a parent class has already declared.
  *
  * @see \Drupal\Tests\KernelTestBase::enableModules()
  * @see \Drupal\Tests\KernelTestBase::bootKernel()
  *
  * @var array
  */
 protected static $modules = [];

You'll need to match this declaration in your own code (though you can add to the array).