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/tls_test.go

@@ -294,7 +294,7 @@ func TestTLSDialerFailureSplitHostPort(t *testing.T) {
 func TestTLSDialerFailureDialing(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel() // immediately fail
-	dialer := TLSDialer{Dialer: &net.Dialer{}}
+	dialer := TLSDialer{Dialer: defaultDialer}
 	conn, err := dialer.DialTLSContext(ctx, "tcp", "www.google.com:443")
 	if err == nil || !strings.HasSuffix(err.Error(), "operation was canceled") {
 		t.Fatal("not the error we expected", err)