* Read data validations for drop down list in another sheet.
* Add function testLoadXlsxDataValidationOfAnotherSheet() in class tests/PhpSpreadsheetTests/Reader/XlsxTest.php for unit test.
* Add sample xlsx for unit tests.
* Modifiy call function isset() for warnings.
* Additional assertions to ensure that the worksheet has been read correctly for DataValidation that references a list on a different worksheet
* This should resolve the phpstan issues
Co-authored-by: Mark Baker <mark@lange.demon.co.uk>
Olivier TARGET