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.
Temporarily replace them with getISOFields().calendar/timeZone just to
keep the tests running until we remove Calendar and TimeZone objects
altogether.
See: tc39/proposal-temporal#2826