Fix test bootstrap autoloader

The previous `require_once` statement returns `true` when it loads the
autoload file rather than an instance of `ClassLoader`. Changing the
statement to `require` returns the instance needed to add the Monolog
test directory.

tests/bootstrap.php

@@ -9,5 +9,5 @@
  * file that was distributed with this source code.
  */
 
-$loader = require_once __DIR__ . "/../vendor/autoload.php";
+$loader = require __DIR__ . "/../vendor/autoload.php";
 $loader->add('Monolog\\', __DIR__);