How about this, will this work?

Author: harm-less

composer.json

@@ -27,7 +27,9 @@
 	},
 	"autoload-dev": {
 		"psr-4": {
-			"PE\\": "tests/PE"
+			"PE\\Tests\\": "tests/PE/Tests",
+			"PE\\Nodes\\": "tests/PE/nodes",
+			"PE\\Samples\\": "tests/PE/samples"
 		}
 	}
 }

tests/bootstrap.php

@@ -8,6 +8,4 @@
 define('ROOT_DIR', dirname(TESTS_DIR));
 
 // Load our autoloader, and add our Test class namespace
-$autoloader = require(ROOT_DIR . '/vendor/autoload.php');
-$autoloader->add('PE\\nodes', __DIR__ . 'nodes');
-$autoloader->add('PE\\samples', __DIR__ . 'samples');
+$autoloader = require(ROOT_DIR . '/vendor/autoload.php');