We did this inconsistently in PlainDate and ZonedDateTime, and not in the
other calendar-carrying types. Additionally, we don't have to create the
calendar explicitly in PlainDate.
Each from() method except Calendar and TimeZone should test that when you
pass an instance of that type, the return value is a clone of that
instance, and a distinct object.
These tests existed already for some types; regularize them and add the
ones that didn't exist yet.
In order to test the referenceISODay of a PlainYearMonth we add an
argument to TemporalHelpers.assertPlainYearMonth.
