go/packages/packagestest: fix MustCopyFileTree so that file fragments are always slash form

Change-Id: I866384c2b382cbf3e839f0ef826eea654319fa5d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/176219
Run-TryBot: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

go/packages/packagestest/export.go

@@ -263,7 +263,7 @@ func MustCopyFileTree(root string) map[string]interface{} {
 		if err != nil {
 			return err
 		}
-		result[fragment] = Copy(path)
+		result[filepath.ToSlash(fragment)] = Copy(path)
 		return nil
 	}); err != nil {
 		log.Panic(fmt.Sprintf("MustCopyFileTree failed: %v", err))