go/internal/gccgoimporter: add missing copyright notice

Change-Id: I2a1d5132f7263d8dc0e2a42c42b37b9d962a3ab7
Reviewed-on: https://go-review.googlesource.com/c/142892
Reviewed-by: Alan Donovan <adonovan@google.com>

go/internal/gccgoimporter/testenv_test.go

@@ -1,3 +1,7 @@
+// 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.
+
 package gccgoimporter
 
 // This file contains testing utilities copied from $GOROOT/src/internal/testenv/testenv.go.