test(date): validate locale formats via runtime expansion

[naoNao89]

Tried to reproduce on macOS 26 but tests pass fine locally.

Fails on CI's macOS 15.7.2 though. Turns out nl_langinfo returns different format codes across macOS versions. Fixed by validating the expanded output instead of checking literal format strings.

Fixes #9654

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/sort/sort-debug-keys is no longer failing!

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/sort/sort-stale-thread-mem. tests/sort/sort-stale-thread-mem is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

Note: The gnu test tests/pr/pr-tests was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/tty/tty-eof. tests/tty/tty-eof is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/date-locale-hour. tests/date/date-locale-hour is passing on 'main'. Maybe you have to rebase?

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/date/date-locale-hour. tests/date/date-locale-hour is passing on 'main'. Maybe you have to rebase?

[codspeed-hq]

Merging this PR will not alter performance

✅ 284 untouched benchmarks
⏩ 38 skipped benchmarks¹

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_{Comparing naoNao89:fix/macos-locale-tests-9654 (3fcd82e) with main (682d9c5)²}

Footnotes

1. 38 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (c7ad6ff) during the generation of this report, so 682d9c5 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cp/cp-mv-enotsup-xattr. tests/cp/cp-mv-enotsup-xattr is passing on 'main'. Maybe you have to rebase?
Note: The gnu test tests/csplit/csplit-io-err was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tty/tty-eof is no longer failing!

[naoNao89]

passed:

locale::tests::test_default_format_contains_valid_codes
locale::tests::test_locale_format_structure

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/pr/bounded-memory is no longer failing!