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.
https://github.com/tc39/proposal-temporal/pull/1907 was a bug that caused
negative Duration strings with fractional units to be rounded incorrectly.
Add tests that ensure the rounding mode is correct.
This was a normative change that achieved consensus at the December 2021
TC39 meeting.