[SimpleDiskAndNetworkTest] don't terminate the Runtime during the test

Before 05dc56ed the two test cases testDiskAndNetwork, testIov
where combined in one function which was terminating the runtime after the client
closed its socketafter the test was done.
This was already broken before 05dc56ed but after the separation of the
two use cases the race between the Runtime terminating and the execution of
the last parts of the test-runner's alphaFiber became bigger.

When the Runtime terminates because the last worker running the testIoV
code has outstanding IO the test-runner's successSem is never notified.