switch to using .status.startTimestamp for sorting backups

Signed-off-by: Steve Kriss <steve@heptio.com>

pkg/util/test/test_backup.go

@@ -130,3 +130,8 @@ func (b *TestBackup) WithFinalizers(finalizers ...string) *TestBackup {
 
 	return b
 }
+
+func (b *TestBackup) WithStartTimestamp(startTime time.Time) *TestBackup {
+	b.Status.StartTimestamp = metav1.Time{Time: startTime}
+	return b
+}