Fix failing tests in TestMachineManager

When the machine manager calls the _onGlobalContainerChanged(), it calls updateNumberExtruders
Enabled, which triggers the signal numberExtrudersEnabledChanged. This, in turn, triggers the need
to check the MachineManager's pyqtProperty numberExtrudersEnabled. Now, since this property has no
setter, it cannot be patched. Instead, to work properly, patch the updateNumberExtrudersEnabled.

CURA-7827

tests/TestMachineManager.py

@@ -21,6 +21,7 @@ def machine_manager(application, extruder_manager, container_registry, global_st
     application.getGlobalContainerStack = MagicMock(return_value = global_stack)
     with patch("cura.Settings.CuraContainerRegistry.CuraContainerRegistry.getInstance", MagicMock(return_value=container_registry)):
         manager = MachineManager(application)
+        manager.updateNumberExtrudersEnabled = MagicMock()
         manager._onGlobalContainerChanged()
 
     return manager