fix(oohelperd): enforce timeout for each measurement step (#888)

While working on https://github.com/ooni/probe/issues/2237, I noticed
there's no enforced timeout for measurement tasks.

So, this diff introduces the following timeouts:

1. use a 4 seconds timeout for the DNS lookup;

2. use a 10 seconds timeout for TCP;

3. use a 15 seconds timeout for HTTP.

They are a bit stricter than what we have on the probe because the TH
should supposedly have better bandwidth and connectivity.

internal/cmd/oohelperd/dns.go

@@ -7,6 +7,7 @@ package main
 import (
 	"context"
 	"sync"
+	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
 	"github.com/ooni/probe-cli/v3/internal/model"
@@ -38,6 +39,9 @@ type dnsConfig struct {
 
 // dnsDo performs the DNS check.
 func dnsDo(ctx context.Context, config *dnsConfig) {
+	const timeout = 4 * time.Second
+	ctx, cancel := context.WithTimeout(ctx, timeout)
+	defer cancel()
 	defer config.Wg.Done()
 	reso := config.NewResolver()
 	defer reso.CloseIdleConnections()