composer.json
composer.lock
phpunit.xml
    vendor/
    tests/
        functional/
            BaseTestCase.php
            HomepageTest.php

{
"require": {
    "php": ">=5.5.0",
    "slim/slim": "^3.1",
    "slim/php-view": "^2.0",
    "monolog/monolog": "^1.17"
},
"require-dev": {
    "phpunit/phpunit": ">=4.8 < 6.0"
},
"autoload-dev": {
    "psr-4": {
        "Tests\": "tests/"
    }
}
}

<?xml version="1.0" encoding="utf-8" ?>
<phpunit colors="true" bootstrap="vendor/autoload.php">
    <testsuites>
        <testsuite name="Initial tests">
            <directory>tests/</directory>
        </testsuite>
    </testsuites>
</phpunit>

<?php
namespace TestsFunctional;

use SlimApp;
use SlimHttpRequest;
use SlimHttpResponse;
use SlimHttpEnvironment;

class BaseTestCase extends PHPUnit_Framework_TestCase
{
  ...

<?php
namespace TestsFunctional;

class HomepageTest extends BaseTestCase
{
   ...

$ composer update

Loading composer repositories with package information
Updating dependencies (including require-dev)
Nothing to install or update
Writing lock file
Generating autoload files

$ vendor/bin/phpunit

PHP Fatal error:  Class 'TestsFunctionalBaseTestCase' not found in <project-root>/tests/functional/HomepageTest.php on line 6

$ composer dump-autoload

Generating autoload files
sfbagency@sfb1:~/clients/ctest/dev$

...
Tests\' => array($baseDir . '/tests'),
...

src/
tests/
      Models/
             ModelOneTest.php
             ModelTwoTest.php <---
      Fixtures/
             Fixtures.php