test: add test for unlowered ITab

See #15604.  This was a bug in a CL that has since been
rolled back.  Adding a test to challenge the next attempter.

Change-Id: Ic43be254ea6eaab0071018cdc61d9b1c21f19cbf
Reviewed-on: https://go-review.googlesource.com/23000Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue15604.go

+// compile
+
+// Copyright 2016 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 bug
+
+import "os"
+
+func f(err error) {
+	var ok bool
+	if err, ok = err.(*os.PathError); ok {
+		if err == os.ErrNotExist {
+		}
+	}
+}