Simplify start/stop helper func in system tests

The system test should never attempt to start or stop any other server than those that belong to that system test. Therefore, it is not necessary to specify the system test name in function calls.

Additionally, this makes it possible to run the test inside a differently named directory, as its name is automatically detected with the $SYSTESTDIR variable. This enables running the system tests inside a temporary directory.

Direct use of stop.pl was replaced with a more systematic approach to use stop_servers helper function.