refactor(dialer): it should close idle connections (#457)

Like we did before for the resolver, a dialer should propagate the
request to close idle connections to underlying types.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/mocks/dialer.go

@@ -7,10 +7,16 @@ import (
 
 // Dialer is a mockable Dialer.
 type Dialer struct {
-	MockDialContext func(ctx context.Context, network, address string) (net.Conn, error)
+	MockDialContext          func(ctx context.Context, network, address string) (net.Conn, error)
+	MockCloseIdleConnections func()
 }
 
 // DialContext calls MockDialContext.
 func (d *Dialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	return d.MockDialContext(ctx, network, address)
 }
+
+// CloseIdleConnections calls MockCloseIdleConnections.
+func (d *Dialer) CloseIdleConnections() {
+	d.MockCloseIdleConnections()
+}

internal/netxlite/mocks/dialer_test.go

@@ -7,7 +7,7 @@ import (
 	"testing"
 )
 
-func TestDialerWorks(t *testing.T) {
+func TestDialerDialContext(t *testing.T) {
 	expected := errors.New("mocked error")
 	d := Dialer{
 		MockDialContext: func(ctx context.Context, network string, address string) (net.Conn, error) {
@@ -23,3 +23,16 @@ func TestDialerWorks(t *testing.T) {
 		t.Fatal("expected nil conn")
 	}
 }
+
+func TestDialerCloseIdleConnections(t *testing.T) {
+	var called bool
+	d := &Dialer{
+		MockCloseIdleConnections: func() {
+			called = true
+		},
+	}
+	d.CloseIdleConnections()
+	if !called {
+		t.Fatal("not called")
+	}
+}