Add tests for ECMA-402 PR #444

[sffc]

tc39/ecma402#444 is a small PR filling in a gap in the spec. We should ensure there is test coverage for it.

[FrankYFTang]

https://github.com/tc39/test262/blob/main/test/intl402/RelativeTimeFormat/constructor/supportedLocalesOf/length.js

[FrankYFTang]

I think we should close this issue because https://github.com/tc39/test262/blob/main/test/intl402/RelativeTimeFormat/constructor/supportedLocalesOf/length.js should already test this since Jun 26, 2018 after @Ms2ger added it

[ryzokuken]

+1 on closing the issue, I cannot close it myself because I don't have access to this repo.