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Unit testing can prove to be challenging for a multichain setup with Foundry. Hence, we’ve provided a lightweight test environment MockHyperlaneEnvironment for you to unit test your cross-chain app while avoiding the need to fork multiple networks. Most multichain apps will be built on top of our Mailbox contract. So, we’ve abstracted away the details of a deployed mailbox with a MockMailbox and our environment contains an originMailbox and a destinationMailbox on the same chain. Internally, we store the messages arriving to the destination in the inboundMessages mapping on the destination mailbox. We simulate message delivery by enqueuing messages and increment the inboundProcessedNonce with MockMailbox.processNextInboundMessage(). The setup for the simple messaging forge test is as following:

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Sending a message

contract SimpleMessagingTest is Test {
    // origin and destination domains (recommended to be the chainId)
    uint32 origin = 1;
    uint32 destination = 2;

    // both mailboxes will be on the same chain but different addresses
    MockMailbox originMailbox;
    MockMailbox destinationMailbox;

    // contract which can receive messages
    TestRecipient receiver;

    function setUp() public {
        originMailbox = new MockMailbox(origin);
        destinationMailbox = new MockMailbox(destination);
        originMailbox.addRemoteMailbox(destination, destinationMailbox);

        receiver = new TestRecipient();
    }

    function testSendMessage() public {
        string _message = "Aloha!";
        originMailbox.dispatch(
            destination,
            TypeCasts.addressToBytes32(address(receiver)),
            bytes(_message)
        );
        // simulating message delivery to the destinationMailbox
        destinationMailbox.processNextInboundMessage();
        assertEq(string(receiver.lastData()), _message);
    }
}

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Testing Router-based apps

Assuming you’re testing TestCrosschainApp which inherits from Router: 
contract CrosschainAppTest is Test {
    // origin and destination domains (recommended to be the chainId)
    uint32 origin = 1;
    uint32 destination = 2;

    function setUp() public {
        environment = new MockHyperlaneEnvironment(origin, destination);

        // your cross-chain app
        TestCrosschainApp originTelephone  = new TestCrosschainApp(environment.mailboxes(origin));
        TestCrosschainApp destinationTelephone  = new TestCrosschainApp(environment.mailboxes(destination));

        // assuming you're inheriting the Router pattern from https://github.com/hyperlane-xyz/hyperlane-monorepo/blob/main/solidity/contracts/client/Router.sol
        originTelephone.enrollRemoteRouter(destinationTelephone);
        destinationTelephone.enrollRemoteRouter(originTelephone);
    }
}
Call processNextPendingMessage() and processNextPendingMessageFromDestination() to process inbound messages for destination and origin mailboxes respectively. Now, you can make your cross-chain calls from origin to destination and vice-versa: 
    function testRemoteTelephoneCallFromOrigin() public {
        // check behavior on origin
        vm.expectEmit(true, true, true, false);
        emit TelephoneRinging(destination,  TypeCasts.bytes32ToAddress(destinationTelephone), "Hello!"); // example event on origin
        originTelephone.callRemote(destination,  TypeCasts.bytes32ToAddress(destinationTelephone), "Hello!");

        // simulating message delivery origin -> destination
        environment.processNextPendingMessage();

        // check behavior on destination
        assertEq(destinationTelephone.latestMessage(originTelephone) == "Hello!");
    }

    function testRemoteTelephoneCallFromDestination() public {
        // check behavior on destination
        vm.expectEmit(true, true, true, false);
        emit TelephoneRinging(origin,  TypeCasts.bytes32ToAddress(originTelephone), "Howdy!"); // example event on destination
        destinationTelephone.callRemote(origin,  TypeCasts.bytes32ToAddress(originTelephone), "Howdy!");

         // simulating message delivery destination -> origin
        environment.processNextPendingMessageFromDestination();

        // check behavior on origin
        assertEq(originTelephone.latestMessage(destinationTelephone) == "Howdy!");
    }
If you want to use your own ISM for your app, you can override the defaultIsm mailbox provides by passing it to the Router’s initialize method like following: 
contract CrosschainAppTest is Test {
    // origin and destination domains (recommended to be the chainId)
    uint32 origin = 1;
    uint32 destination = 2;

    function setUp() public {
        ...

        TestIgp igp = new TestIgp(); // example InterchainGasPaymaster passed as the hook

        // deploy your own ISM contracts to verify messages between originTelephone and destinationTelephone
        TelephoneISM originIsm = new TelephoneISM(); // local ISM for origin
        TelephoneISM destinationIsm = new TelephoneISM(); // local ISM for destination


        originTelephone.initialize(address(igp), address(originIsm), msg.sender);
        originTelephone.initialize(address(igp), address(destinationIsm), msg.sender);

        ...
    }
}
You can find examples of our unit testing setup here: InterchainAccountRouterTest.