test: add test for fixed issue 7863

Fixes #7863

LGTM=rsc
R=rsc, ruiu
CC=golang-codereviews
https://golang.org/cl/98610045

test/fixedbugs/issue7863.go

+// run
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import "time"
+
+func main() {
+	now := time.Now()
+	f := now.Unix
+	if now.Unix() != f() {
+		println("BUG: ", now.Unix(), "!=", f())
+	}
+}