As event_obj._length doesn't return the number of days when the selection spans
multiple weeks, the test was wrong when the selection ended on the first day of
a subsequent week.

This fix was originaly written by rha-odoo at 95d344b, but I rewrote it a little.
I would have liked a cleaner way of finding how many days there were between the
two dates, but I couldn't find anything better, considering I didn't want to
create new objects just for that test.
Fixes opw 614703.