Use correct variable for unit tests

Up until now I was using the variable $_SERVER["PHP_SELF"] to perform
Python-style unit testing: if a script has been run directly at the
command line using e.g. "php Loco.php", then the unit tests are
executed, otherwise not. Unfortunately it looks like this approach is
useless if somebody is executing the PHP script from a server of some
kind, and also the "PHP_SELF" variable isn't documented. From my own
experiments it looks like the variable we want is "SCRIPT_FILENAME".
Documentation: www.php.net/manual/en/reserved.variables.server.php