Migrate to new testing library

[thekid]

See xp-framework/rfc#344 and https://github.com/xp-framework/test. Mostly migrated automatically, needed a small fix for imports from global namespace (use Traversable;), using patterns inside #[Expect] which previously supported substring matches and adjustments to not use fn()-syntax inside #[Condition].

Metrics

Before

Tests:       902 passed, 4 skipped
Memory used: 12261.99 kB (12314.85 kB peak)
Time taken:  0.231 seconds

After

Test cases:  902 succeeded, 4 skipped
Memory used: 11046.30 kB (11100.43 kB peak)
Time taken:  0.248 seconds (1.403 seconds overall)