util: add DontTouch trait with dontTouchPorts method

src/main/scala/system/TestHarness.scala

@@ -14,6 +14,7 @@ class TestHarness()(implicit p: Parameters) extends Module {
   val dut = Module(LazyModule(new ExampleRocketSystem).module)
   dut.reset := reset | dut.debug.ndreset
 
+  dut.dontTouchPorts()
   dut.tieOffInterrupts()
   dut.connectSimAXIMem()
   dut.connectSimAXIMMIO()