fix(ooniprobe): use ooniprobe-cli-unattended for unattended runs (#714)

This diff changes the software name used by unattended runs for which
we did not override the default software name (`ooniprobe-cli`).

It will become `ooniprobe-cli-unattended`. This software name is in line
with the one we use for Android, iOS, and desktop unattended runs.

While working in this diff, I introduced string constants for the run
types and a string constant for the default software name.

See https://github.com/ooni/probe/issues/2081.

internal/engine/probeservices/checkin_test.go

@@ -17,7 +17,7 @@ func TestCheckInSuccess(t *testing.T) {
 		Platform:        "android",
 		ProbeASN:        "AS12353",
 		ProbeCC:         "PT",
-		RunType:         "timed",
+		RunType:         model.RunTypeTimed,
 		SoftwareName:    "ooniprobe-android",
 		SoftwareVersion: "2.7.1",
 		WebConnectivity: model.OOAPICheckInConfigWebConnectivity{
@@ -54,7 +54,7 @@ func TestCheckInFailure(t *testing.T) {
 		Platform:        "android",
 		ProbeASN:        "AS12353",
 		ProbeCC:         "PT",
-		RunType:         "timed",
+		RunType:         model.RunTypeTimed,
 		SoftwareName:    "ooniprobe-android",
 		SoftwareVersion: "2.7.1",
 		WebConnectivity: model.OOAPICheckInConfigWebConnectivity{

internal/engine/session.go

@@ -238,7 +238,7 @@ func (s *Session) KibiBytesSent() float64 {
 //
 // - ProbeCC: if empty, set to Session.ProbeCC();
 //
-// - RunType: if empty, set to "timed";
+// - RunType: if empty, set to model.RunTypeTimed;
 //
 // - SoftwareName: if empty, set to Session.SoftwareName();
 //
@@ -270,7 +270,7 @@ func (s *Session) CheckIn(
 		config.ProbeCC = s.ProbeCC()
 	}
 	if config.RunType == "" {
-		config.RunType = "timed" // most conservative choice
+		config.RunType = model.RunTypeTimed // most conservative choice
 	}
 	if config.SoftwareName == "" {
 		config.SoftwareName = s.SoftwareName()

internal/engine/session_internal_test.go

@@ -107,7 +107,7 @@ func TestSessionCheckInSuccessful(t *testing.T) {
 	if mockedClnt.Config.ProbeCC != "IT" {
 		t.Fatal("invalid Config.ProbeCC")
 	}
-	if mockedClnt.Config.RunType != "timed" {
+	if mockedClnt.Config.RunType != model.RunTypeTimed {
 		t.Fatal("invalid Config.RunType")
 	}
 	if mockedClnt.Config.SoftwareName != "miniooni" {