test: test issue 7884 (already fixed)

I don't know when the bug was fixed, but empirically it was.
Make sure it stays fixed by adding a test.

Fixes #7884.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/93500043

test/fixedbugs/issue7884.go

+// compile
+
+// 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 "fmt"
+
+func main() {
+	var ii interface{} = 5
+	zz, err := ii.(interface{})
+	fmt.Println(zz, err)
+}