In Apex Testing > Test Apex Triggers tutorial,

I would like to run TestAccountDeletion but I encounter the following error:

Methods defined as TestMethod do not support Web service callouts

The code copied and pasted from the tutorial:

@isTest
private class TestAccountDeletion {
    @isTest static void TestDeleteAccountWithOneOpportunity() {
        // Test data setup
        // Create an account with an opportunity, and then try to delete it
        Account acct = new Account(Name='Test Account');
        insert acct;
        Opportunity opp = new Opportunity(Name=acct.Name + ' Opportunity',
                                       StageName='Prospecting',
                                       CloseDate=System.today().addMonths(1),
                                       AccountId=acct.Id);
        insert opp;
        
        // Perform test
        Test.startTest();
        
        Database.DeleteResult result = Database.delete(acct, false);
        
        Test.stopTest();
        
        // Verify 
        // In this case the deletion should have been stopped by the trigger,
        // so verify that we got back an error.
        System.assert(!result.isSuccess());
        System.assert(result.getErrors().size() > 0);
        System.assertEquals('Cannot delete account with related opportunities.',
                             result.getErrors()[0].getMessage());
    }
}

 

I'm doing the simple lightning components challenge and have hit this problem in my existing trailhead org and a brand new dev org that I've just created:

Challenge not yet complete... here's what's wrong: 
There was an unexpected error while verifying this challenge. Usually this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: QVWBQHAG