add decent method to get the programs return to interpreter

only had such methods in tests, but they really belong in code
also adding fake values to true ,false and nil

test/risc/test_interpreter.rb

@@ -38,6 +38,10 @@ module Risc
     def test_pos
       assert_equal 1 , @interpreter.clock
     end
+    def test_return_value
+      @interpreter.run_all
+      assert_equal 5 , @interpreter.get_sys_return
+    end
   end
   class TestInterpreterTicks < MiniTest::Test
     include Ticker
@@ -98,7 +102,7 @@ module Risc
     def test_tick_15 #more than a binary code worth
       15.times {@interpreter.tick}
     end
-    class TestInterpreterDies #< MiniTest::Test
+    class TestInterpreterDies < MiniTest::Test
       include Ticker
       def setup
         @string_input = as_main("random.call")