Fix server shutdown in bin/tests/system/run.sh

a4f02819 is a flawed backport of cf510593 - it retained the original invocation of bin/tests/system/stop.pl in bin/tests/system/run.sh. This results in the former script being called twice for each system test, which does not cause problems on Unix systems, but triggers false positives about named instances dying prematurely on Windows. Fix by removing the offending invocation of bin/tests/system/stop.pl from bin/tests/system/run.sh.