feat: implement the unattended group (#183)

We don't want to run performance in the background because this
causes too much traffic towards m-lab servers.

When we'll have the check-in API, this will be the entry point we'll
use to contact such an API and get things to do.

Part of https://github.com/ooni/probe/issues/1289.

internal/nettests/run.go

@@ -44,7 +44,7 @@ func RunGroup(config RunGroupConfig) error {
 		return err
 	}
 
-	group, ok := NettestGroups[config.GroupName]
+	group, ok := All[config.GroupName]
 	if !ok {
 		log.Errorf("No test group named %s", config.GroupName)
 		return errors.New("invalid test group name")