Add new root CA to the signal test (#977)

All measurements collected since 2022-10-19 with previous versions
of OONI Probe will wrongly report sfu.voip.signal.org as blocked
as it switched to using a different root CA

This fixes: https://github.com/ooni/probe/issues/2344

internal/engine/experiment/signal/signal_test.go

@@ -17,7 +17,7 @@ func TestNewExperimentMeasurer(t *testing.T) {
 	if measurer.ExperimentName() != "signal" {
 		t.Fatal("unexpected name")
 	}
-	if measurer.ExperimentVersion() != "0.2.0" {
+	if measurer.ExperimentVersion() != "0.2.1" {
 		t.Fatal("unexpected version")
 	}
 }