Test RANK(), not the underlying floating point representation of duration

The RANK() results depended on the floating point representation
of time differences (durations), which for visually equal MM:SS
display values don't have to be equal if similar durations result
from different start and end times. Change that to a well defined
duration in seconds.

Change-Id: Ib69a6b43f48abd2d4fe605021acfd2800e75b5e1
Reviewed-on: https://gerrit.libreoffice.org/71926Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins