Debug flag allowing overwriting stairs

And using it in the movement tests, so running doesn't stop
at stairs.

src/nhlua.c

@@ -974,7 +974,9 @@ nhl_doturn(lua_State *L)
 }
 
 /* set debugging flags. debugging use only, of course. */
-/* nh.debug_flags({ mongen = false, hunger = false }); */
+/* nh.debug_flags({ mongen = false,
+                    hunger = false,
+                    overwrite_stairs = true }); */
 static int
 nhl_debug_flags(lua_State *L)
 {
@@ -1004,6 +1006,12 @@ nhl_debug_flags(lua_State *L)
         iflags.debug_hunger = !(boolean)val; /* value in lua is negated */
     }
 
+    /* allow overwriting stairs */
+    val = get_table_boolean_opt(L, "overwrite_stairs", -1);
+    if (val != -1) {
+        iflags.debug_overwrite_stairs = (boolean)val;
+    }
+
     return 0;
 }