tests: Add missing 'embeds' field to wp_mail test case

Author: SirLouen

tests/phpunit/tests/pluggable/wpMail.php

@@ -446,6 +446,7 @@ public function test_phpmailer_exception_thrown() {
 			'message'                  => 'Test Message',
 			'headers'                  => array(),
 			'attachments'              => array(),
+			'embeds'                   => array(),
 			'phpmailer_exception_code' => 2,
 		);