Without custom calendars and time zones there are actually a bunch of
things that we now can't test on implementations that don't have non-ISO
calendars or non-UTC time zones. (Alternatively, we can say that these are
functionalities that those implementations don't have to implement.)
I discovered these tests had omitted some arguments to the PlainDateTime
constructor, leaving the calendar in the wrong position.
The tests were technically not incorrect, since the operation
ToIntegerThrowOnInfinity on the string "gregory" gives 0. But they could
spuriously pass if the implementation didn't do argument conversion
correctly, failed to throw on eraYear being ±Infinity, but subsequently
threw RangeError anyway because the calendars of the arguments didn't
match.