test: :white_check_mark: add tests crate `db`

Pull Request type

Resolves: #NA

Trait Implementations:
- Implemented the Default trait on primitives to allow for the creation of default instances during testing.
- Implemented Eq and PartialEq traits on primitives to enable equality checks within tests.
Unit Tests:
- Block Header Storage and Retrieval:
- Added tests to ensure that block headers are correctly stored and can be accurately retrieved.
- Transaction Storage and Retrieval:
- Added tests to validate the storage of transactions and their subsequent retrieval, ensuring that the data remains consistent.
- Receipts Storage and Retrieval:
- Introduced tests for storing and retrieving receipts, confirming that all relevant details are preserved.
- Transaction Search:
- Implemented tests to verify that the transaction search functionality works as expected, returning the correct transaction data based on provided search criteria.
Pending Block Management:
- Added tests to cover the management of pending blocks, ensuring that pending block is handled properly in various scenarios, such as during storage and retrieval processes.