In order to test this functionality on hosts that don't have Intl, we have
to use "UTC" and offset-only time zones here, as the full set of IANA time
zone names are not required to be supported across all hosts.
This makes some progress on https://github.com/tc39/test262/issues/3253
Unfortunately, in #3304 I made a last-minute mistake when I added the
uncallable value to the assertion message, and neglected to test it;
Symbols can't be converted to strings like that, so these tests would
fail. This fixes the assertion messages.
Tests for the normative changes made to Temporal in
https://github.com/tc39/proposal-temporal/pull/1829
In a previous version of the specification, there was a fallback to the
intrinsic getOffsetNanosecondsFor when it was undefined.
