fix the exit sequence

was returning wrong register (1, not 0)
also saving the message for test, which changes order as the return value destroys the message

test/support/risc_interpreter.rb

@@ -35,8 +35,8 @@ module Risc
     end
 
     def get_return
-      assert_equal Parfait::Message , @interpreter.get_register(:r0).class
-      @interpreter.get_register(:r1)
+      assert_equal Parfait::Message , @interpreter.get_register(:r8).class
+      @interpreter.get_register(:r0)
     end
 
     # do as many as given ticks in the main, ie main_at more