Workaround for bug (?) in ProcessorScheduler
"Due to some bug (?) in ProcessorScheduler, we need to set
exitWhenNoMoreUserProcesses to prevent process dispatcher
to exit prematurely.
Sometimes, ProcessorScheduler>>checkForEndOfSispatch thought
there's no runnable user proless even though test runner did not
finish (was merely waiting for external command to finish).
Happened sometimes. not always.