[xharness] Don't throw NRE if a simulator doesn't exist.

We automatically create simulators when needed, but it won't work if the
simulator runtime isn't installed. So handle the case where a test might not
have a simulator to execute in correctly.

tests/xharness/Jenkins.cs

@@ -503,7 +503,7 @@ namespace xharness
 				await tv.FindSimulatorAsync ();
 
 			var rv = new List<AggregatedRunSimulatorTask> ();
-			foreach (var taskGroup in testVariations.GroupBy ((RunSimulatorTask task) => task.Device.UDID)) {
+			foreach (var taskGroup in testVariations.GroupBy ((RunSimulatorTask task) => task.Device?.UDID ?? task.Candidates.ToString ())) {
 				rv.Add (new AggregatedRunSimulatorTask (taskGroup) {
 					Jenkins = this,
 					TestName = $"Tests for {taskGroup.Key}",

tests/xharness/Simulators.cs

@@ -352,6 +352,11 @@ namespace xharness
 			object lock_obj = new object ();
 			Task<SimDevice []> findTask;
 
+			public override string ToString ()
+			{
+				return $"Simulators for {Target} (MinVersion: {MinVersion})";
+			}
+
 			public IEnumerator<SimDevice> GetEnumerator ()
 			{
 				return new Enumerator ()