Test trendline properties using an emport -> ixport cycle

Added a test which checks the preservation of properties for
trendlines / regression curves in an export -> import cycle
using different file formats - ODS, XLS and XLSX.

Change-Id: I59fe6c045f7f503ee074e6a2741fa017756b3018
Reviewed-on: https://gerrit.libreoffice.org/6877
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>