test: add test for issue8347

Fixes #8347.

LGTM=dave
R=golang-codereviews, dave
CC=golang-codereviews
https://golang.org/cl/109600044

test/fixedbugs/issue8347.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
+
+func main() {
+	c := make(chan bool, 1)
+	ok := true
+	for i := 0; i < 12; i++ {
+		select {
+		case _, ok = <-c:
+			if i < 10 && !ok {
+				panic("BUG")
+			}
+		default:
+		}
+		if i < 10 && !ok {
+			panic("BUG")
+		}
+		if i >= 10 && ok {
+			close(c)
+		}
+	}
+}