bin/tests/system/stop.pl does not wait for named's lock file to be cleaned up

bin/tests/system/stop.pl only waits for the PID file to be cleaned up, but that is not the last thing named does during shut down - the lock file is cleaned up after the PID file. Thus, in some rare cases, stop.pl may consider a named instance to be fully shut down when in fact it is not. If the lock file is still in place when a given named instance is restarted using start.pl --noclean --restart, its startup will fail, yielding a false positive for the given system test.

An example of such a failure can be found here (for the xfer system test): https://gitlab.isc.org/isc-projects/bind9/-/jobs/196548