Previously getISOFields() was used to get the exact value of the
[[Calendar]] and [[TimeZone]] internal slots, as well as to get the
reference ISO year for PlainMonthDay and reference ISO day for
PlainYearMonth.
Use calendarId and timeZoneId for the former and toString() for the
latter.
In many cases we created a TimeZone or Calendar instance from a built-in
time zone or calendar. These tests can be trivially adapted to just use
the string ID.
In several tests involving custom calendars, we need to change the
implementation of dateFromFields/monthDayFromFields/yearMonthFromFields so
that the returned object gets the receiver as its calendar after chaining
up to the builtin implementation.
Normative PR: https://github.com/tc39/proposal-temporal/pull/2482
