ical-recurrence: test that a weekly 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.
weeklyDateTimeRecurrence has its 'd <= limit' guard inside the list
that filterSetPos numbers, so in the week that straddles the limit the
candidate set is truncated and BYSETPOS picks from the truncated set.
monthlyDateTimeRecurrence 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 the week of the 20th of January 2020 is Friday the
24th, which lies beyond a limit of the 22nd, so the recurrence set up to
that limit contains no instance in that week at all.