second specification, second requirement, business rule for gold customer

testSuiteDiscountRateByMembershipType.php

<?php
function test_with_gold_member_will_discount_ten_percent() {
    if ( 0.1 === discountRateByMembershipType('gold') {
        echo 'Test Done';
    } else {
        echo 'Test Fail';
    }
}
//... code test_with_platinum_member_will_discount_fifteen_percent here