Add a test for NatController.

Bug: 9580643
Change-Id: I6ac3b754ec0b720674c6221e3a776314e86fe58c
diff --git a/server/NatController.h b/server/NatController.h
index f23bf84..500385b 100644
--- a/server/NatController.h
+++ b/server/NatController.h
@@ -47,6 +47,10 @@
     int runCmd(int argc, const char **argv);
     int setForwardRules(bool set, const char *intIface, const char *extIface);
     int setTetherCountingRules(bool add, const char *intIface, const char *extIface);
+
+    // For testing.
+    friend class NatControllerTest;
+    static int (*execFunction)(int, char **, int *, bool, bool);
 };
 
 #endif