fdo#71350 add Excel 2010 function CONFIDENCE.NORM and CONFIDENCE.T

Change-Id: I341b0c7a61047627aac1f12bc4653b6f9e65b2e3
Reviewed-on: https://gerrit.libreoffice.org/6625
Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>