Issue #3265429 by neclimdul, eleonel: Fix array keys in MailManagerTest

(cherry picked from commit 0bf085bb)

core/tests/Drupal/Tests/Core/Mail/MailManagerTest.php

@@ -129,7 +129,7 @@ protected function setUpMailManager($interface = []) {
   public function testGetInstance() {
     $interface = [
       'default' => 'php_mail',
-      'default' => 'test_mail_collector',
+      'example_testkey' => 'test_mail_collector',
     ];
     $this->setUpMailManager($interface);