timer: Implement the shutdown for GNU Smalltalk too.

Timer.st

@@ -106,6 +106,12 @@ bit difficult to do this race free.'>
 
     doShutDown [
         "Nothing for GST"
+        loop ifNil: [^self].
+        quit := true.
+        loop isSuspended ifTrue: [loop resume].
+        delay ifNotNil: [:the_delay | the_delay signal].
+        processExited wait.
+        Transcript nextPutAll: 'Stopped the TimerScheduler process'; cr
     ]
 
     dispatchTimers [