Tests for the normative changes made to Temporal in
https://github.com/tc39/proposal-temporal/pull/1873
This adds a new Temporal helper calendar that asserts that its dateAdd()
method is always called with a PlainDate instance. This allows testing
that relativeTo parameters are always converted to PlainDate if they are
not ZonedDateTime and not undefined. Prior to the normative PR, they
would be converted to PlainDateTime instead.
Additionally and optionally, the helper calendar can also assert that its
dateAdd() method is called with a specific PlainDate instance. This allows
testing that the instance is the same PlainDate passed as the relativeTo
parameter (in the case of Duration methods) or is the receiver (in the
case of PlainDate methods). For the PlainDateTime and PlainYearMonth
methods the PlainDate instance is synthesized internally so there is no
need to assert that dateAdd() is called with a specific instance.
