I face the same issue. My build works well, but in recent days, when it runs in xenial as default, I always get error messge:
amysqladmin: connect to server at ‘localhost’ failed
for command mysqladmin create woocommerce_test --user=root --password= --host=localhost --protocol=tcp
My solution is force it use trusty by add dist:trusty into .yml file. But if there’s any way to make it work in xenial, please let me know.