Add tests for prohibited operations on extruder stacks

These operations are explicitly prohibited, so they should raise an exception.

Contributes to issue CURA-3497.

tests/Settings/TestExtruderStack.py

@@ -0,0 +1,28 @@
+# Copyright (c) 2017 Ultimaker B.V.
+# Cura is released under the terms of the AGPLv3 or higher.
+
+import pytest #This module contains automated tests.
+import unittest.mock #For the mocking and monkeypatching functionality.
+
+import cura.Settings.ExtruderStack #The module we're testing.
+from cura.Settings.Exceptions import InvalidOperationError #To check whether the correct exceptions are raised.
+
+##  An empty extruder stack to test with.
+@pytest.fixture()
+def extruder_stack() -> cura.Settings.ExtruderStack.ExtruderStack:
+    return cura.Settings.ExtruderStack.ExtruderStack
+
+##  Tests whether adding a container is properly forbidden.
+def test_addContainer(extruder_stack):
+    with pytest.raises(InvalidOperationError):
+        extruder_stack.addContainer(unittest.mock.MagicMock())
+
+##  Tests whether inserting a container is properly forbidden.
+def test_insertContainer(extruder_stack):
+    with pytest.raises(InvalidOperationError):
+        extruder_stack.insertContainer(0, unittest.mock.MagicMock())
+
+##  Tests whether removing a container is properly forbidden.
+def test_removeContainer(extruder_stack):
+    with pytest.raises(InvalidOperationError):
+        extruder_stack.removeContainer(unittest.mock.MagicMock())