[xharness] Attach a native debugger when running watchOS tests on device. (#560)

Attaching a native debugger prevents the watch from backgrounding
the app, and makes the test run able to complete without having
to babysit it.

mk/xamarin.mk

@@ -1,5 +1,5 @@
 ifdef ENABLE_XAMARIN
-NEEDED_MACCORE_VERSION := 2ff8293d6e2a0d37a7bf784cdd41b079afa531dd
+NEEDED_MACCORE_VERSION := 395484fb8831a870089b857fb4cce5ccb31136f8
 NEEDED_MACCORE_BRANCH := master
 
 MACCORE_DIRECTORY := maccore

tests/xharness/AppRunner.cs

@@ -474,6 +474,12 @@ namespace xharness
 
 				args.Append (" --launchdev");
 				args.AppendFormat (" \"{0}\" ", launchAppPath);
+
+				var waits_for_exit = false;
+				if (mode == "watchos") {
+					args.Append (" --attach-native-debugger"); // this prevents the watch from backgrounding the app.
+					waits_for_exit = true;
+				}
 				
 				AddDeviceName (args);
 
@@ -489,9 +495,11 @@ namespace xharness
 
 				listener.StartAsync ();
 				main_log.WriteLine ("Starting test run");
-				// This will not wait for app completion
-				await ProcessHelper.ExecuteCommandAsync (Harness.MlaunchPath, args.ToString (), main_log, TimeSpan.FromMinutes (1));
-				if (listener.WaitForCompletion (TimeSpan.FromMinutes (Harness.Timeout))) {
+
+				double launch_timeout = waits_for_exit ? Harness.Timeout : 1;
+				double listener_timeout = waits_for_exit ? 0.2 : Harness.Timeout;
+				await ProcessHelper.ExecuteCommandAsync (Harness.MlaunchPath, args.ToString (), main_log, TimeSpan.FromMinutes (launch_timeout));
+				if (listener.WaitForCompletion (TimeSpan.FromMinutes (listener_timeout))) {
 					main_log.WriteLine ("Test run completed");
 				} else {
 					main_log.WriteLine ("Test run did not complete in {0} minutes.", Harness.Timeout);