Modernize benchmark-document-loading script

 - use argparse instead of getopt
 - use with statement for opening files
 - refactor writeReport function to reduce copypaste
 - various fixes to make code more pythonic

Change-Id: I6569dfc856040e1e75b550135f4092cc27bef7f5
Reviewed-on: https://gerrit.libreoffice.org/43644Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>