[test] add mock test for client package

[allencloud]

Ⅰ. Issue Description

For more reliable client package which could be a golang SDK for Pouch developer, we should add more guarantee work on this.

When adding mock test for Pouch's client package, we could refer to PR #965 which could be regarded to be an example.

To be more specific, when doing mock test for API calling function in package client, we should add a mock server function in HTTPClient, then we simulate the action of server in the mock function.

Like:

func TestContainerStart(t *testing.T) {
	expectedURL := "/containers/container_id/start"

	httpClient := newMockClient(func(req *http.Request) (*http.Response, error) {
		if !strings.HasPrefix(req.URL.Path, expectedURL) {
			return nil, fmt.Errorf("Expected URL '%s', got '%s'", expectedURL, req.URL)
		}
		if req.Header.Get("Content-Type") == "application/json" {
			var startConfig interface{}
			if err := json.NewDecoder(req.Body).Decode(&startConfig); err != nil {
				return nil, fmt.Errorf("failed to parse json: %v", err)
			}
		}

		return &http.Response{
			StatusCode: http.StatusOK,
			Body:       ioutil.NopCloser(bytes.NewReader([]byte(""))),
		}, nil
	})

	client := &APIClient{
		HTTPCli: httpClient,
	}

	if err := client.ContainerStart(context.Background(), "container_id", ""); err != nil {
		t.Fatal(err)
	}
}

In that newMockClient(func(req *http.Request) (*http.Response, error) is to simulate the handler of a server. But actually it is in the client side.

So when we call client.ContainerStart, the request will be dealt by newMockClient.

Help Wanted

There are still many API calling functions which misses such kind of mock test. Help wanted for the solving. 👍 🛩 🦁 🍺

[allencloud]

Could you help to check whether this work has been all done by contributors? If not, could you keep to work on this? @ZouRui89