composer phpunit psr-4 autoload class not found

Namespace directories are case sensitive. You have to rename the folder to Functional

As said in the PSR-4 documentation:

The subdirectory name MUST match the case of the sub-namespace names.

I had a similar problem but without syntax problem. As soon as I added a second ClassTest (ModelTwoTest.php below), the Fixtures.php class was autoloaded. If I remove 1 modelTest, only the remaining ModelTest is autoloaded without Fixtures. Weird behavior.

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