Do not enforce reading the configuration from files.

The www/_include.php script, included by all scripts in www/, checks unconditionally for the existence of the config.php file. However, this prevents us from testing the scripts automatically. Instead of checking for the file, we just try to load the configuration, and live with it if it works. That way we can pre-load the configuration using SimpleSAML_Configuration::loadFromArray(), as we are doing in some tests.