Fatal error when running Acquia Agent tests: Call to undefined function _acquia_agent_authenticator()

When running the Acquia Agent tests, I am receiving a fatal error: Call to undefined function _acquia_agent_authenticator(). The reason is that I do not have the Acquia Agent module installed when running the tests, therefore the unit tests don't have access to the _acquia_agent_authenticator() function. This will likely be an issue with the test bot on D.O. According the to the DrupalUnitTestCase::setUp() docblocks:

Unlike DrupalWebTestCase::setUp(), DrupalUnitTestCase::setUp() does not install modules because tests are performed without accessing the database. Any required files must be explicitly included by the child class setUp() method.

Therefore we should explicitly require the acquia_agent.module file in the overridden setUp() method so tests can be run without having to install the module on the testing environment.