[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 poperly 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.

Fixes #25 (closed). Even if I don't know why #25 (closed) happens.