[test-runner] gracefully terminate the runtime

The successSem was a hack used to ensure tests succeed even if the
runtime does not terminate. But our runtime now properly terminates
and thus the successSem can be replaces with a Runtime::initiateTermination
and Runtime::waitUntilFinished pair.

This seams more correct, fixes that the SimpleDiskAndNetwork test hangs
on its successSem in release builds and makes RuntimeDestroyTest obsolete.