PhpUnit testing repositories Symfony findOneBy

I am net to phpunit in Symfony 3 and I'm wondering what exactly should I follow when testing a repository. For example, I have the following repo function:

/**
 * {@inheritdoc}
 */
public function findAdminRole()
{
    $role = $this->repository->findOneBy(['name' => Role::ADMIN]);

    if (null === $role) {
        throw new NotFoundException();
    }

    return $role;
}

What exactly a test for this would look like? Should I test that the function findOneBy and the NotFoundException are called or to get real data values? I am kinda stuck here.

Thank you.