MVP of a signal messenger test (#230)

* MVP of a signal messenger test

* Add minimal signal test unit tests

* Add Signal test to the im nettest group

* Add test for https://sfu.voip.signal.org/

* Fix bug in client-side determination of blocking status

* Add uptime.signal.org to the test targets

* Add more tests

* Check for invalid CA being passed
* Check that the update function works as expected

* Update internal/engine/experiment/signal/signal_test.go

Co-authored-by: Simone Basso <bassosimone@gmail.com>

* fix: back out URL we shouldn't have changed

When merging probe-engine into probe-cli, we changed too many URLs
and some of them should not have been changed.

I noticed this during the review of Signal and I choose to add
this commit to revert such changes.

While there, make sure the URL of the experiment is OK.

* fix(signal): reach 100% of coverage

Just so that we can focus on areas of the codebase where we need
more coverage, let us avoid missing an easy line to test.

Co-authored-by: Simone Basso <bassosimone@gmail.com>

cmd/ooniprobe/internal/nettests/groups.go

@@ -35,6 +35,7 @@ var All = map[string]Group{
 		Label: "Instant Messaging",
 		Nettests: []Nettest{
 			FacebookMessenger{},
+			Signal{},
 			Telegram{},
 			WhatsApp{},
 		},

cmd/ooniprobe/internal/nettests/signal.go

@@ -0,0 +1,17 @@
+package nettests
+
+// Signal test implementation
+type Signal struct {
+}
+
+// Run starts the test
+func (h Signal) Run(ctl *Controller) error {
+	builder, err := ctl.Session.NewExperimentBuilder(
+		"signal",
+	)
+	if err != nil {
+		return err
+	}
+
+	return ctl.Run(builder, []string{""})
+}