Everywhere an ISO string is accepted in Temporal, a seconds value of :60
should always be coerced to :59, because of how leap seconds are handled
in ISO strings.
In property bags, a 'seconds: 60' property is not subject to that rule: it
should be handled according to the overflow option if there is one.
These tests existed already for some types; regularize them and add the
ones that didn't exist yet.
Philip Chimento