Important new functionality has been added to test your smart contracts: the concordium-smart-contract-testing library makes it possible to create and run automatic integration tests of smart contracts. This will allow a smart contract developer to write code that runs multiple contracts in a locally-controlled environment, interacts with them, and asserts that the eventual output and state of the contracts are as expected. For more information about how to enable this, see Integration test a contract in Rust.
Mainnet release notes
Testnet release notes