Fix tests by adding required return type hints

tests/Integration/Api/Application/ApplicationApiIntegrationTestCase.php

@@ -31,7 +31,7 @@ abstract class ApplicationApiIntegrationTestCase extends IntegrationTestCase
      * Bootstrap application API tests. Creates a default admin user and associated API key
      * and also sets some default headers required for accessing the API.
      */
-    public function setUp()
+    public function setUp(): void
     {
         parent::setUp();