HandleBadMethodsTest.cls

@isTest 
private class HandleBadMethodsTest {
    
    @isTest
    static void testCustomException() {
        //validate that our method will reject negative integers

        Boolean exceptionWasThrown = false;

        try {
            SampleBadMethods.iAddUpPositiveIntegers(-1,5);
        } catch (SampleBadMethods.SampleCustomException e) {
            //This assert verifies the correct error message was thrown
            System.assertEquals(e.getMessage(), 'I only take positive integers!');
            exceptionWasThrown = true;
        }

        //This assert verifies that an exception was indeed thrown.  The above assert is insufficient, 
        //because the test would pass even if no exception was thrown at all
        System.assertEquals(exceptionWasThrown, true);
    }

}