Magium offers a library for helping you ensure that with whichever action you are testing the expected email address is being added to the proper mailing list.

Getting started with magium/mailchimp is very easy.

composer require magium/mailchimp

Then in your test assert that the email address is either subscribed or not subscribed, depending on the action.

class AssertSubscriberTest extends AbstractTestCase

    public function testIsSubscribedTest()
        $assertion = $this->getAssertion(Subscribed::ASSERTION);
        $assertion->setEmail('[email protected]');