Add missing unit testing page to the sidebar in the Sway Book
closes
- [Storage Vectors](./common-collections/storage_vec.md)
- [Storage Maps](./common-collections/storage_map.md)
- [Testing](./testing/index.md)
+ - [Unit Testing](./testing/unit-testing.md)
- [Testing with Rust](./testing/testing-with-rust.md)
- [Application Frontend](./frontend/index.md)
- [TypeScript SDK](./frontend/typescript_sdk.md)
