yii2/apps/advanced/frontend/tests/functional/LoginCept.php

<?php

use common\tests\_pages\LoginPage;

$I = new TestGuy($scenario);
$I->wantTo('ensure login page works');

$loginPage = LoginPage::openBy($I);

$I->amGoingTo('submit login form with no data');
$loginPage->login('', '');
$I->expectTo('see validations errors');
$I->see('Username cannot be blank.', '.help-block');
$I->see('Password cannot be blank.', '.help-block');

$I->amGoingTo('try to login with wrong credentials');
$I->expectTo('see validations errors');
$loginPage->login('admin', 'wrong');
$I->expectTo('see validations errors');
$I->see('Incorrect username or password.', '.help-block');

$I->amGoingTo('try to login with correct credentials');
$loginPage->login('erau', 'password_0');
$I->expectTo('see that user is logged');
$I->see('Logout (erau)', 'ul.nav li a');
$I->dontSee('Login', 'ul.nav li a');
$I->dontSee('Signup', 'ul.nav li a');