drupal_get_path function for third party PHP includes files
drupal_get_path() returns the path of a module, a theme, a theme engine, or a profile. It's not for loading a generic PHP file, for which you need module_load_include() which (despite the name) can be used for a PHP file that can be loaded with require_once.

Supposing you add the soap-wsse.php file in the includes directory of the test_module module, you cal load that file using the following code.

module_load_include('php', 'test_module', 'includes/soap-wsse');