public function login(Request $request)
{
    $user = new User;
    $rules_login = $user->rules;
    $rules_login['email'] = '';
    $rules_login['roles'] = '';
    $rules_login['date_of_birth'] = '';
    $rules_login['country_id'] = '';
    $rules_login['username'] = 'required|max:255|regex:/^[a-z0-9_]+$/';
    $validator = Validator::make($request->all(), $rules_login);
    if ($validator->fails()) {
        return $this->jsonResponse(['errors' => $validator->errors()], 400);
    } else {

Route::post('login', 'AuthLoginController@login');

public function testLoginEmpty()
{
    $this->post('/login', ['username' => '', 'password' => '1234567890'])->response
        ->seeStatusCode(400);
    $this->post('/login', ['username' => 'qwerty', 'password' => ''])
        ->seeStatusCode(400);
    $this->post('/login', ['username' => '', 'password' => ''])
        ->seeStatusCode(400);
}