bug 56557: open test-data/spreadsheet/56557.xlsx such that the unit test test56557...

bug 56557: open test-data/spreadsheet/56557.xlsx such that the unit test test56557() does not leave a modified file on disk after running.

I verified that
> Workbook wb = XSSFTestDataSamples.openSampleWorkbook("56557.xlsx")
and
> Workbook wb = WorkbookFactory.create(XSSFTestDataSamples.getSampleFile("56557.xlsx"), password=null, readOnly=false);
both fail when the fix from r1798913 for SXSSFWorkbook is not applied and both pass when the fix is applied.

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1799304 13f79535-47bb-0310-9956-ffa450edef68