Add `SimHarness.RunMillis(t)` function

For now, this is just a wrapper around RunCycles(n), but the two could diverge
in the future. RunMillis(t) should always cause `t` milliseconds to elapse in
the simulator, regardless of how many cycles that takes.

Signed-off-by: Michael Richters <gedankenexperimenter@gmail.com>

testing/SimHarness.h

@@ -29,6 +29,9 @@ class SimHarness {
  public:
   void RunCycle();
   void RunCycles(size_t n);
+  void RunMillis(size_t t) {
+    RunCycles(t);
+  }
 
   void Press(KeyAddr key_addr);
   void Release(KeyAddr key_addr);