Issue #1925930 by dawehner: Document the container variable on TestBase class

core/modules/simpletest/lib/Drupal/simpletest/TestBase.php

@@ -161,6 +161,13 @@ abstract class TestBase {
    */
   public $dieOnFail = FALSE;
 
+  /**
+   * The dependency injection container used in the test.
+   *
+   * @var \Symfony\Component\DependencyInjection\ContainerInterface
+   */
+  protected $container;
+
   /**
    * Constructor for Test.
    *