go/zodb: persistent tests: Use correct % for refcnt

Persistent.refcnt is int, %s prints it as e.g.

	%!s(int=1)

-> use %d.

go/zodb/persistent_test.go

@@ -100,7 +100,7 @@ func checkObj(t testing.TB, obj IPersistent, jar *Connection, oid Oid, serial Ti
 		badf("invalid state: %s  ; want %s", pbase.state, state)
 	}
 	if pbase.refcnt != refcnt {
-		badf("invalid refcnt: %s  ; want %s", pbase.refcnt, refcnt)
+		badf("invalid refcnt: %d  ; want %d", pbase.refcnt, refcnt)
 	}
 	if pbase.instance != obj {
 		badf("base.instance != obj")