test: assert splits_of preserves creation order

[Sam-Rytech]

Adds a test to assert that IDs returned by splits_of follow their creation order across several creates by one creator.

• Implements splits_of_preserves_creation_order test in contracts/splitter/src/test.rs.
• Fixed a duplicated #![cfg(test)] attribute in test.rs to satisfy clippy CI check.
• Validated via cargo test, cargo clippy, and npm run build. All checks pass.

Closes #100