Update LoadTest.php

Closes #3378742 by defining return type setUp(): void

tests/src/Functional/LoadTest.php

@@ -29,7 +29,12 @@ class LoadTest extends BrowserTestBase {
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
+  protected $defaultTheme = 'stark';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp(): void {
     parent::setUp();
     $this->user = $this->drupalCreateUser(['administer site configuration']);
     $this->drupalLogin($this->user);