fix: gate Iceberg test utilities behind the integration feature

[alexanderbianchi]

Summary

Gate Iceberg's test_utils with #[cfg(any(feature = "integration", test))], matching the main DFD crate. Production builds no longer compile the fixture include_str! expressions, so consumers can vendor only iceberg/ without the sibling test data.

• Require integration for the six integration-test modules that import these helpers; unit tests retain access through cfg(test).
• Keep fixtures and their paths unchanged under testdata/iceberg/. This replaces the earlier fixture-relocation approach.
• Update the documented test command and feature description. Callers importing test_utils must now enable integration.

No codec/runtime or dependency changes. Integration tests still use the repository fixtures; this does not make feature-enabled crate-only copies self-contained.