Add some unit-tests for Mid vb function (includes fdo#73771)

Change-Id: Ib5ee18207897cbe16a818096769cdb055dc9dad7
Reviewed-on: https://gerrit.libreoffice.org/7583
Reviewed-by: Noel Power <noel.power@suse.com>
Tested-by: Noel Power <noel.power@suse.com>