frontend\tests\FunctionalTester::seeValidationError PHP Method

    public function seeValidationError($message)
    {
        $this->see($message, '.help-block');
    }

 public function signupWithEmptyFields(FunctionalTester $I)
 {
     $I->see('Signup', 'h1');
     $I->see('Please fill out the following fields to signup:');
     $I->submitForm($this->formId, []);
     $I->seeValidationError('Username cannot be blank.');
     $I->seeValidationError('Email cannot be blank.');
     $I->seeValidationError('Password cannot be blank.');
 }