UserSaveTest.php

<?php

namespace Drupal\Tests\user\Kernel;

use Drupal\KernelTests\KernelTestBase;
use Drupal\user\Entity\User;

/**
 * Tests account saving for arbitrary new uid.
 *
 * @group user
 */
class UserSaveTest extends KernelTestBase {
    
    /**
     * {@inheritdoc}
     */
    protected static $modules = [
        'system',
        'user',
    ];
    
    /**
     * Ensures that an existing password is unset after the user was saved.
     */
    public function testExistingPasswordRemoval() {
        $this->installSchema('system', [
            'sequences',
        ]);
        $this->installEntitySchema('user');
        
        /** @var \Drupal\user\Entity\User $user */
        $user = User::create([
            'name' => $this->randomMachineName(),
        ]);
        $user->save();
        $user->setExistingPassword('existing password');
        $this->assertNotNull($user->pass->existing);
        $user->save();
        $this->assertNull($user->pass->existing);
    }

}