os: pass tests on Plan 9 again

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5563046
diff --git a/src/pkg/os/stat_plan9.go b/src/pkg/os/stat_plan9.go
index 8d3b8a8..f731e43 100644
--- a/src/pkg/os/stat_plan9.go
+++ b/src/pkg/os/stat_plan9.go
@@ -97,3 +97,8 @@
 func Lstat(name string) (FileInfo, error) {
 	return Stat(name)
 }
+
+// For testing.
+func atime(fi FileInfo) time.Time {
+	return time.Unix(int64(fi.(*FileStat).Sys.(*Dir).Atime), 0)
+}