fix(ptx/obfs4_test.go): avoid context-vs-normal-code race

We want to test whether we get the context failure if the error
generated inside normal code happens _after_ the context cancellation.

The best way to do that is to write code that is not racy. To this
end, we just need to pause normal code until we know that the context
has returned to the caller. We also need to ensure we do not leak
a goroutine, hence we use a WaitGroup to check that.

Fixes ooni/probe#1750

internal/ptx/obfs4_test.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"net"
 	"strings"
+	"sync"
 	"testing"
 
 	"github.com/ooni/probe-cli/v3/internal/atomicx"
@@ -65,14 +66,17 @@ func TestOBFS4DialerFailsWithConnectionErrorAndNoContextExpiration(t *testing.T)
 func TestOBFS4DialerFailsWithConnectionErrorAndContextExpiration(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	expected := errors.New("mocked error")
+	unexpected := errors.New("mocked error")
 	o4d := DefaultTestingOBFS4Bridge()
+	sigch := make(chan interface{})
+	wg := &sync.WaitGroup{}
+	wg.Add(1)
 	o4d.UnderlyingDialer = &netxmocks.Dialer{
 		MockDialContext: func(ctx context.Context, network string, address string) (net.Conn, error) {
-			// We cancel the context before returning the error, which makes
-			// the context cancellation happen before us returning.
 			cancel()
-			return nil, expected
+			<-sigch
+			wg.Done()
+			return nil, unexpected
 		},
 	}
 	conn, err := o4d.DialContext(ctx)
@@ -82,6 +86,8 @@ func TestOBFS4DialerFailsWithConnectionErrorAndContextExpiration(t *testing.T) {
 	if conn != nil {
 		t.Fatal("expected nil conn here")
 	}
+	close(sigch)
+	wg.Wait()
 }
 
 // obfs4connwrapper allows us to observe that Close has been called