public function testClass()
 {
     $model = new User();
     $this->assertFalse($model->isValid());
     $this->assertTrue($model->hasErrors());
     $this->assertTrue($model->hasErrors('username'));
     $this->assertEquals(['username' => ['Cannot be empty', 'Minimal length is 3']], $model->getErrors());
     $this->assertEquals(['Cannot be empty', 'Minimal length is 3'], $model->getErrors('username'));
     $model->clearErrors('username');
     $this->assertEquals([], $model->getErrors());
 }