The default input definition of the Console Component requires a command argument (s. Symfony\Component\Console\Application::getDefaultInputDefinition()). When you try to execute a command which has no arguments using the CommandTester you have to pass a command argument (with an arbitrary value) (s. example Gist). Otherwise, you receive a RuntimeException (see also symfony/symfony-docs#2870):

RuntimeException: Not enough arguments in Symfony\Component\Console\Input\Input on line 76