CN: simpler test of pointer eqs

This simpler test shows how alloc-ids change the behaviour of CN.
The assertion that pointer->integer->pointer conversion leaves a
pointer unchanged is now false, and was previously provable. A
more interesting question is, should this assertion be provable?

tests/cn/test_pointer_eq.c

@@ -0,0 +1,6 @@
+
+
+void f (int *p)
+{
+  /*@ assert (p == ((pointer) ((integer) p))); @*/
+}