Fix strftime directives handling and tighten test assertions

- Remove incorrect %C mapping (was mapping century to full year)
- Add special handling for %C (century), %c (datetime), %n (newline), %t (tab), %x (date)
- Add %h mapping (same as %b for abbreviated month)
- Tighten edge-case test assertions to use assertSame/assertMatchesRegularExpression
- Add tests for new directives: %C, %c, %n, %t, %x, %h
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