Temporal doesn't specify concrete era names, so tests shouldn't assert
for example that the era code of the current Gregorian era is `"ce"`. We
still want to validate the era names somehow however, so allow alternative
era names using the era codes from the "Intl era and monthCode" proposal.
As in the previous commit, without custom calendars and time zones, some
functionality cannot be tested on implementations that don't have any
other calendars and time zones than iso8601 and UTC.
Some of the staging tests fall in this category. We take the opportunity
to port these into proper tests, in the intl402/ folder.
