test(imagepush): 开只读窗口遇 409 时像 GC Job 一样重试，消除 -race 下的偶发失败

	maint := httptest.NewServer(g.MaintHandler())

	t.Cleanup(maint.Close)

	host := strings.TrimPrefix(gate.URL, "http://")

	// The gate counts a write as finished only after its handler returns, which

	// is just after the client already has the response, so a window opened

	// right behind the last push can see it in flight for a moment and answer

	// 409. The registry GC Job retries until it gets 200 (platform/workloads.go); so

	// does this helper.

	setReadOnly := func(on bool) {

		t.Helper()

		path := "/readwrite"

		if on {

			path = "/readonly?lease=600"

		}

		deadline := time.Now().Add(5 * time.Second)

		for {

			resp, err := http.Post(maint.URL+path, "text/plain", nil)

			if err != nil {

				t.Fatal(err)

			}

			resp.Body.Close()

		if resp.StatusCode != http.StatusOK {

			if resp.StatusCode == http.StatusOK {

				return

			}

			if !on || resp.StatusCode != http.StatusConflict || time.Now().After(deadline) {

				t.Fatalf("POST %s = %d", path, resp.StatusCode)

			}

			time.Sleep(10 * time.Millisecond)

		}

	}

	ref := host + "/user-uploads/sub-gc:latest"