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{