laravel has a test to check that its inbuilt login functionality works - if you go to github -> tests and see how it does it.
I know why you are getting this because normally to post you need a CSRF token which you dont have.. actually i have looked at my test
$user = factory(User::class)
->states('client')
->create();
$this->post('/login', ['email' => $user->email, 'password' => 'secret'])
->assertLocation('/clients/dashboard');
and i dont have anything about CSRF even in the verifyCSRF middleware :/