Pull in additional coverage for String.localeCompare

[schuay]

The current coverage seems fairly basic and is missing many relevant special cases. I'd suggest to pull in at JSC's https://github.com/WebKit/WebKit/blob/main/JSTests/stress/string-localeCompare.js.

Besides that, it may make sense to add further coverage.

cc @mathiasbynens @FrankYFTang

[mathiasbynens]

cc @jugglinmike @anba

[jugglinmike]

Improving coverage always sounds good to me!

Here's a few tips for anyone who wants to take up the cause:

• in recognition of the optional nature of ECMA402, Test262 maintains tests for the String.prototype.localeCompare method in two different places: test/intl402/String/prototype/localeCompare/ and test/built-ins/String/prototype/localeCompare/. New tests should go in the former directory if they concern normative text in ECMA402, and they should go in the latter if they concern normative text in ECMA262.
• the referenced JSC stress test seems like a great source of inspiration. To make its assertions useful for conformance testing, please separate them into distinct files whose description metadata explains their intent