// we write a file to the cache indicating that all test results from before
		// right now are to be ignored.
		dir, _ := cache.DefaultDir()
		if dir != "off" {
			f, err := lockedfile.Edit(filepath.Join(dir, "testexpire.txt"))
			if err == nil {
				now := time.Now().UnixNano()
				buf, _ := io.ReadAll(f)
				prev, _ := strconv.ParseInt(strings.TrimSpace(string(buf)), 10, 64)
				if now > prev {