Bug 11417: make sure remove_unused_authorities.pl accepts --test

Bug 11417: make sure remove_unused_authorities.pl accepts --test

This patches adds support for the --test option, as well as a
short message telling the user the script is running in test mode.

Test plan :
- Launch the script with -h to see the help
- Launch the script with --test and --aut with an authtypecode
  that is used in your instance
- Make sure it does the same thing as launching it with -t
- Launch the script for real and make sure it still works as
  expected, deleting unused authorities.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 6657860010ef27792251596590c1471ad5d08ad8)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>