yii2-bootstrap/apps/basic/tests/functional/LoginCept.php

<?php
use tests\functional\_pages\LoginPage;

$I = new TestGuy($scenario);
$I->wantTo('ensure that login works');
$loginPage = LoginPage::of($I);

$I->amOnPage(LoginPage::$URL);
$I->see('Login', 'h1');

$I->amGoingTo('try to login with empty credentials');
$loginPage->login('', '');
$I->expectTo('see validations errors');
$I->see('Username cannot be blank.');
$I->see('Password cannot be blank.');

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

$I->amGoingTo('try to login with correct credentials');
$loginPage->login('admin', 'admin');
$I->expectTo('see user info');
$I->see('Logout (admin)');