Don't append Unicode extensions to a default locale that already has one

[jessealama]

Fixes #615.

testNumberFormat and nine standalone tests build locales by appending strings like "-u-<key>-<value>" to the default locale coming from resolvedOptions(). When a host reports a default locale that already carries a Unicode extension sequence (e.g. "en-US-u-va-posix"), the result has two "-u-" substrings. This causes the constructor to throw a RangeError before these tests reach the behavior they're meant to check. JSC does this in CI, where 43 tests currently fail for that reason.

This PR adds a new getLocaleBaseName helper to testIntl.js. It strips extension and private-use subtags (thereby mirroring Intl.Locale.p.baseName). The helper is then applied at each site before any extension gets appended.

Whether the default locale may contain an extension at all is a separate question, covered by a new conformance test in #5125.

The helper uses plain string juggling rather than Intl.Locale. That's arguably a more proper solution, but using Intl.Locale would add a feature dependency to legacy Collator and NumberFormat tests. I also notice that V8's baseName currently returns "en-US-u-va-posix" unchanged for exactly this input, so the helper wouldn't do anything there. Switching to Intl.Locale once both concerns are resolved would be a reasonable follow-up.

[github-actions]

9 new or modified tests were run on 7 engines.

View workflow run

Test	Engine262	GraalJS	JavaScriptCore	QuickJS	SpiderMonkey	V8	XS
intl402/Collator/ignore-invalid-unicode-ext-values.js	❌	✅	✅	❌	✅	✅	❌
intl402/Collator/numeric-and-caseFirst.js	❌	✅	✅	❌	✅	✅	❌
intl402/DateTimeFormat/casing-numbering-system-calendar-options.js	❌	✅	✅	❌	✅	✅	❌
intl402/DateTimeFormat/numbering-system-calendar-options.js	❌	✅	✅	❌	✅	✅	❌
intl402/DateTimeFormat/prototype/resolvedOptions/resolved-locale-with-hc-unicode.js	❌	✅	✅	❌	✅	✅	❌
intl402/NumberFormat/casing-numbering-system-options.js	❌	✅	✅	❌	✅	✅	❌
intl402/NumberFormat/numbering-system-options.js	❌	✅	✅	❌	✅	✅	❌
intl402/NumberFormat/test-option-currency.js	❌	✅	✅	❌	✅	✅	❌
intl402/NumberFormat/throws-for-currency-style-without-currency-option.js	❌	✅	✅	❌	✅	✅	❌

[ptomato]

I'm not positive about what the right move is here. This seems to work fine. But on the other hand, it seems simpler (and arguably more robust) to insert the extension sequences in the right place with an addUExtension(key, value) helper function:

• If there's no -u-, append -u-${key}-${value}.
• If there is -u- and no other single-letter subtags after it, append -${key}-${value}.
• If there is -u- and any other single-letter subtag after it, insert -${key}-${value}- right before the first such single-letter subtag.