test: add test to verify that string copies don't get optimized away

Fixes #25834

Change-Id: I33e58dabfd04b84dfee1a9a3796796b5d19862e7
Reviewed-on: https://go-review.googlesource.com/118295
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/strcopy.go

+// run
+
+// Copyright 2018 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.
+
+// Test that string([]byte(string)) makes a copy and doesn't reduce to
+// nothing. (Issue 25834)
+
+package main
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+func main() {
+	var (
+		buf      = make([]byte, 2<<10)
+		large    = string(buf)
+		sub      = large[10:12]
+		subcopy  = string([]byte(sub))
+		subh     = *(*reflect.StringHeader)(unsafe.Pointer(&sub))
+		subcopyh = *(*reflect.StringHeader)(unsafe.Pointer(&subcopy))
+	)
+	if subh.Data == subcopyh.Data {
+		panic("sub and subcopy have the same underlying array")
+	}
+}