Common functionality for overridden kernels.

Note: when using this trait, also implement ConfigAwareInterface/ConfigAwareTrait.

CliTestTrait provides an execute() method that is useful for launching executable programs in functional tests.

DrushTestTrait provides a drush() method that may be used to write functional tests for Drush extensions.

OutputUtilsTrait provides some useful utility methods for test classes that define getOutputRaw() and getErrorOutputRaw() methods.