Fix for failing tests for Brazilian time zone.

I experienced some failing tests for Brazilian time zone.
The first one (in IndexArbitraries) was simple. In Brazil, DST clock shifts occur at midnight, so some instants generated were invalid.
The second one (in RRule) was trickier: it was intermittent. When counting recurrence backward, a lower bound is estimated. When this bound lands on a DST start date, the date has a clock shift. The generated index then has a wrong hour and calculate occurrences wrong. When this occurs, I set the lower bound to one day earlier.