Add unittests for SfxBroadcaster

Change-Id: I9ecd8fec395eafbdc2675dda9fa25ac32761123c
Reviewed-on: https://gerrit.libreoffice.org/10343
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>