There was 1 error:

1) CaremonkMainSiteBundleTestsControllerSecurityControllerFunctionalTest::testIndex
InvalidArgumentException: The current node list is empty.

$form = $crawler->selectButton('login')->form();

///////////////////////////////////////////////////////////////////////////////
///////////////////////////////////////////////////////////////////////////////
Functional Test
<?php

namespace CaremonkMainSiteBundleTestsController;

use SymfonyBundleFrameworkBundleTestWebTestCase;

class SecurityControllerFunctionalTest extends WebTestCase
{
    public function testIndex()
    {
        $client = static::createClient();

        $crawler = $client->request('GET', '/login');

        $form = $crawler->selectButton('login')->form();
        $form['username'] = 'testActive';
        $form['password'] = 'passwordActive';
    }
}


//////////////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////////////
Twig view


{# src/Acme/SecurityBundle/Resources/views/Security/login.html.twig #}
{% if error %}
    <div>{{ error.message }}</div>
{% endif %}

<form action="{{ path('caremonk_mainsite_login_check') }}" method="post">
    <label for="username">Username:</label>
    <input type="text" id="username" name="_username" value="{{ last_username }}" />

    <label for="password">Password:</label>
    <input type="password" id="password" name="_password" />

    {#
        If you want to control the URL the user is redirected to on success (more details below)
        <input type="hidden" name="_target_path" value="/account" />
    #}

    <button type="submit">login</button>
</form>

$form = $crawler->selectButton('submit my form please')->form();

$client = new Client();
$crawler = $client->request('GET', '/login');

$client = new Client();
$crawler = $client->request('GET', 'http://site.dev/app_dev.php/login');

$crawler->filter('form[name=yourAwesomeFormName]')->form();