AccountTriggerHandlerTest_snippet.cls

    // Snippet from AccountTriggerHandlerTest

    ...
    mockDataLayer = new MockDataLayer();
    mockFieldReader = new MockFieldReader();
    ...

    // First, the formula field on the child object:
    mockFieldReader.addValueToField(
        mockDataLayer.childAccount,
        'External_URL__c',
        expectedURL
    );

    // Then, the nonwritable child relationship
    mockFieldReader.addValueToField(
        mockDataLayer.parentAccount, 
        'Child_Accounts__r', 
        new List<Account>{mockDataLayer.childAccount}
    );