feat(webconnectivity@v0.5): flag case where noone resolved any address (#953)

See https://github.com/ooni/probe/issues/2290

While there, notice that in such a case the priority selector would hang because of the WaitGroup, so get rid of the WaitGroup and accept that the priority selector is going to hang around for the whole duration of the measurement in some cases. The cancellable `measurer.go`'s context will cause the priority selector to eventually exit when we return from `measurer.go`'s `Run` method.

internal/experiment/webconnectivity/priority.go

@@ -30,7 +30,6 @@ import (
 	"context"
 	"fmt"
 	"net"
-	"sync"
 	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/model"
@@ -80,7 +79,6 @@ func newPrioritySelector(
 	zeroTime time.Time,
 	tk *TestKeys,
 	logger model.Logger,
-	wg *sync.WaitGroup,
 	addrs []DNSEntry,
 ) *prioritySelector {
 	ps := &prioritySelector{
@@ -107,8 +105,7 @@ func newPrioritySelector(
 			ps.nhttps++
 		}
 	}
-	wg.Add(1)
-	go ps.selector(ctx, wg)
+	go ps.selector(ctx)
 	return ps
 }
 
@@ -151,10 +148,7 @@ func (ps *prioritySelector) permissionToFetch(address string) bool {
 // background goroutine and terminates when [ctx] is done.
 //
 // This function implements https://github.com/ooni/probe/issues/2276.
-func (ps *prioritySelector) selector(ctx context.Context, wg *sync.WaitGroup) {
-	// synchronize with the parent
-	defer wg.Done()
-
+func (ps *prioritySelector) selector(ctx context.Context) {
 	// Implementation note: setting an arbitrary timeout here would
 	// be ~an issue because we want this goroutine to be available in
 	// case the only connections from which we could fetch a webpage