How to write phpunit test for change password feature in symfony 3

How to write unit test for the below code

public function changePasswordAction(Request $request)
{
    $user = $this->get('security.token_storage')->getToken()->getUser();

    $change_password = new ChangePassword();

    $form = $this->createForm(ChangePasswordType::class, $change_password);
    $form->handleRequest($request);

    $data = $form->getData();

    if ($form->isValid()) {        

        $password = $this->get('security.password_encoder')
                ->encodePassword($user, $change_password->getNewPassword());
        $user->setPassword($password);

        $em = $this->getDoctrine()->getManager();
        $em->persist($user);
        $em->flush(); 



        return $this->redirectToRoute('dummy');
    }

    return $this->render(
        'changePassword.html.twig', ['form' => $form->createView()]
    );
}