test: assert unwrapped values in elementary_test and unwrap tests

[Jalst]

Summary

Related to #603.

As noted in #603, tests/util.py::elementary_test previously only asserted isinstance(v.unwrap(), unwrapped_type). Consequently, the test_*_unwrap tests in tests/test_items.py could detect type errors, but not value corruptions (e.g. Bool.unwrap returning not bool(self) would still pass both test_true_unwrap and test_false_unwrap).

This PR closes that testing gap:

• Adds an optional expected parameter to elementary_test (defaulting to a sentinel _DEFAULT = object()), asserting v_unwrapped == expected when provided.
• Passes the expected pure Python value across all item unwrap unit tests (int, float, bool, datetime, str, None, time, date).
• Adds value equality assertions to test_aot_unwrap, test_array_unwrap, and test_abstract_table_unwrap in tests/test_items.py.
• Adds value equality assertion to test_toml_document_unwrap in tests/test_toml_document.py.

Verification

• pytest tests: 1,058 passed.
• Pre-commit hooks (trailing-whitespace, end-of-file-fixer, pyupgrade, ruff, ruff-format): all passed.
• Verified mutation sensitivity: deliberately returning a corrupted unwrapped value fails the test suite as expected.