Strict Types for New Tests

Consistent with work being done in PR PHPOffice#3718.

Author: oleibman

tests/PhpSpreadsheetTests/Cell/DataType2Test.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpOffice\PhpSpreadsheetTests\Cell;
 
 use PhpOffice\PhpSpreadsheet\Cell\DataType;

tests/PhpSpreadsheetTests/Writer/Xlsx/Issue3711Test.php

@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace PhpOffice\PhpSpreadsheetTests\Writer\Xlsx;
 
 use PhpOffice\PhpSpreadsheet\Cell\DataType;