ical-recurrence: test that a monthly BySetPos does not depend on the limit
The limit is an implementation detail of this library, not part of the
recurrence rule, so it must only cut off results. It must not change
which instance BYSETPOS selects.
monthlyDateTimeRecurrence has its 'd <= limit' guard inside the list
that filterSetPos numbers, so in the month that straddles the limit the
candidate set is truncated and BYSETPOS picks from the truncated set.
weeklyDateTimeRecurrence and yearlyDateTimeRecurrence have the same
problem; dailyDateTimeRecurrence does not, because there filterSetPos
only ever spans the times within one day.
The last week day of February 2020 is the 28th, which lies beyond a
limit of the 20th, so the recurrence set up to that limit contains no
instance in February at all.