FOS\UserBundle\Document\UserManager::findUserBy PHP Method

    public function findUserBy(array $criteria)
    {
        return $this->repository->findOneBy($criteria);
    }

 /**
  * @depends testUserNew
  * @depends testUserData
  * @param User $user
  */
 public function testUserDelete(User $user)
 {
     $user = $this->userManager->findUserBy(['username' => $user->getUsername()]);
     $this->userManager->deleteUser($user);
 }