[feature/extension-manager] Don't cache the phpbb_root_path in the ext manager

Otherwise the paths are incorrect from e.g. adm/

PHPBB3-10323

tests/extension/finder_test.php

@@ -23,12 +23,12 @@ class phpbb_extension_finder_test extends phpbb_test_case
 				'foo' => array(
 					'ext_name' => 'foo',
 					'ext_active' => '1',
-					'ext_path' => dirname(__FILE__) . '/ext/foo/',
+					'ext_path' => 'ext/foo/',
 				),
 				'bar' => array(
 					'ext_name' => 'bar',
 					'ext_active' => '1',
-					'ext_path' => dirname(__FILE__) . '/ext/bar/',
+					'ext_path' => 'ext/bar/',
 				),
 			));