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/ooapi/integration_test.go

@@ -5,6 +5,7 @@ import (
 	"testing"
 
 	"github.com/ooni/probe-cli/v3/internal/kvstore"
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/ooapi"
 	"github.com/ooni/probe-cli/v3/internal/ooapi/apimodel"
 )
@@ -19,7 +20,7 @@ func TestWithRealServerDoCheckIn(t *testing.T) {
 		Platform:        "android",
 		ProbeASN:        "AS12353",
 		ProbeCC:         "IT",
-		RunType:         "timed",
+		RunType:         model.RunTypeTimed,
 		SoftwareName:    "ooniprobe-android",
 		SoftwareVersion: "2.7.1",
 		WebConnectivity: apimodel.CheckInRequestWebConnectivity{