Update LoadSheetsOnlyTest.php

Add strict types to this new test, consistent with work being done in PR PHPOffice#3718.

Author: oleibman

tests/PhpSpreadsheetTests/Reader/Xls/LoadSheetsOnlyTest.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpOffice\PhpSpreadsheetTests\Reader\Xls;
 
 use PhpOffice\PhpSpreadsheet\Exception as PhpSpreadsheetException;