fix writes to static $kernel property

Author: xabbuh

Test/KernelTestCase.php

@@ -77,7 +77,7 @@ protected static function bootKernel(array $options = [])
 
         $kernel = static::createKernel($options);
         $kernel->boot();
-        self::$kernel = $kernel;
+        static::$kernel = $kernel;
         static::$booted = true;
 
         $container = static::$kernel->getContainer();