ECMA-402 currently specifies the strings "BC" and "AD", but it makes sense to me to make the test robust for a change to BCE/CE nomenclature in the future.

If I understand correctly, we don't actually need to wait for tc39/ecma402#723 to achieve consensus in order to merge this? The test as written already reflects web reality, and disagrees with the spec text. This PR just improves the test, right? (Edit: never mind; I was looking at only the second commit when I wrote this. Still approved, but we do need to wait for the ECMA-402 PR.)

Thank you!