add compile-time flag to test ignoring device controller shutdown

src/darwin/Framework/CHIP/MTRDeviceController.mm

@@ -311,8 +311,15 @@ - (BOOL)isRunning
     return _cppCommissioner != nullptr;
 }
 
+
+//#define TESTING_IGNORE_MTRDEVICECONTROLLER_SHUTDOWN
+
 - (void)shutdown
 {
+#ifdef TESTING_IGNORE_MTRDEVICECONTROLLER_SHUTDOWN
+    MTR_LOG("****** IGNORING call to shutdown in %@", self);
+    return;
+#else
     MTR_LOG("%@ shutdown called", self);
     if (_cppCommissioner == nullptr) {
         // Already shut down.
@@ -321,6 +328,7 @@ - (void)shutdown
 
     MTR_LOG("Shutting down %@: %@", NSStringFromClass(self.class), self);
     [self cleanupAfterStartup];
+#endif // TESTING_IGNORE_MTRDEVICECONTROLLER_SHUTDOWN
 }
 
 // Clean up from a state where startup was called.