Add return types to internal & magic methods when possible

Author: nicolas-grekas

Tests/Functional/app/AppKernel.php

@@ -79,7 +79,7 @@ protected function build(ContainerBuilder $container)
         $container->register('logger', NullLogger::class);
     }
 
-    public function __sleep()
+    public function __sleep(): array
     {
         return ['varDir', 'testCase', 'rootConfig', 'environment', 'debug'];
     }

Tests/Kernel/ConcreteMicroKernel.php

@@ -64,7 +64,7 @@ public function getLogDir(): string
         return $this->cacheDir;
     }
 
-    public function __sleep()
+    public function __sleep(): array
     {
         throw new \BadMethodCallException('Cannot serialize '.__CLASS__);
     }