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/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" {