refactor(oohelperd): improve tests implementation (#835)

After this diff has landed, we have addressed all the points
originally published at https://github.com/ooni/probe/issues/2134.

internal/cmd/oohelperd/main_test.go

@@ -1,15 +1,82 @@
 package main
 
 import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net/http"
+	"net/url"
+	"strings"
 	"testing"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
 )
 
-func TestSmoke(t *testing.T) {
-	// Just check whether we can start and then tear down the server, so
-	// we have coverage of this code and when we see that some lines aren't
-	// covered we know these are genuine places where we're not testing
-	// the code rather than just places like this simple main.
-	go testableMain()
-	srvcancel()  // kills the listener
-	srvwg.Wait() // joined
+func TestWorkAsIntended(t *testing.T) {
+	// let the kernel pick a random free port
+	*endpoint = "127.0.0.1:0"
+
+	// run the main function in a background goroutine
+	go main()
+
+	// prepare the HTTP request body
+	jsonReq := ctrlRequest{
+		HTTPRequest: "https://dns.google",
+		HTTPRequestHeaders: map[string][]string{
+			"Accept":          {model.HTTPHeaderAccept},
+			"Accept-Language": {model.HTTPHeaderAcceptLanguage},
+			"User-Agent":      {model.HTTPHeaderUserAgent},
+		},
+		TCPConnect: []string{
+			"8.8.8.8:443",
+			"8.8.4.4:443",
+		},
+	}
+	data, err := json.Marshal(jsonReq)
+	runtimex.PanicOnError(err, "cannot marshal request")
+
+	// construct the test helper's URL
+	endpoint := <-srvAddr
+	URL := &url.URL{
+		Scheme: "http",
+		Host:   endpoint,
+		Path:   "/",
+	}
+	req, err := http.NewRequest("POST", URL.String(), bytes.NewReader(data))
+	runtimex.PanicOnError(err, "cannot create new HTTP request")
+
+	// issue the request and get the response
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		t.Fatal("unexpected status code", resp.StatusCode)
+	}
+
+	// read the response body
+	data, err = netxlite.ReadAllContext(context.Background(), resp.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// parse the response
+	var jsonResp ctrlResponse
+	if err := json.Unmarshal(data, &jsonResp); err != nil {
+		t.Fatal(err)
+	}
+
+	// very simple correctness check
+	if !strings.Contains(jsonResp.HTTPRequest.Title, "Google") {
+		t.Fatal("expected the response title to contain the string Google")
+	}
+
+	// tear down the TH
+	srvCancel()
+
+	// wait for the background goroutine to join
+	srvWg.Wait()
 }