In the package that comes the Inco library, an extension to the usual Test Foundry contract is provided.
You can import it like this: 
// SPDX-License-Identifier: MIT
pragma solidity ^0.8;

import {IncoTest} from "@inco/lightning/src/test/IncoTest.sol";

contract YourTest is IncoTest {
    function testSomething() public {
        // Your test code here
    }
}
IncoTest extends Foundry’s Test contract, and exposes useful Inco-specific cheatcodes to test your contract.
If you are extending the setUp function, remember to call super.setUp() at the beginning of your function.

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Example

The following example is present with comments explaining the code here in the lightning-rod template.
// SPDX-License-Identifier: MIT
pragma solidity ^0.8;

import {SimpleConfidentialToken} from "../SimpleConfidentialToken.sol";
import {IncoTest} from "@inco/lightning/src/test/IncoTest.sol";
import {GWEI} from "@inco/shared/src/TypeUtils.sol";

contract TestSimpleConfidentialToken is IncoTest {
    SimpleConfidentialToken token;

    function setUp() public override {
        super.setUp();
        token = new SimpleConfidentialToken();
        token.transfer(alice, fakePrepareEuint256Ciphertext(10 * GWEI));
    }

    function testTransfer() public {
        vm.prank(alice);
        token.transfer(bob, fakePrepareEuint256Ciphertext(1 * GWEI));
        processAllOperations();
        uint256 decryptedBobBalance = getUint256Value(token.balanceOf(bob));
        uint256 decryptedAliceBalance = getUint256Value(token.balanceOf(alice));
        assertEq(decryptedBobBalance, 1 * GWEI);
        assertEq(decryptedAliceBalance, 9 * GWEI);
    }
}
Once you are happy with the state of your contract, you can deploy it as-is without any changes and Inco will react to its ops (as long as you are on a supported network, currently only Base-Sepolia). Read on for the reference of the available cheatcodes.