Fix global stack tests.

Contributes to CURA-5628.

tests/conftest.py

@@ -4,7 +4,7 @@
 # The purpose of this class is to create fixtures or methods that can be shared among all tests.
 
 import pytest
-from UM.Qt.QtApplication import QtApplication #QTApplication import is required, even though it isn't used.
+from UM.Qt.QtApplication import QtApplication #QtApplication import is required, even though it isn't used.
 from cura.CuraApplication import CuraApplication
 from cura.MachineActionManager import MachineActionManager
 
@@ -15,24 +15,9 @@ def application() -> CuraApplication:
     application = CuraApplication.getInstance()
     if application is None:
         application = CuraApplication()
-        application.initialize()
     return application
 
 # Returns a MachineActionManager instance.
 @pytest.fixture()
 def machine_action_manager(application) -> MachineActionManager:
     return application.getMachineActionManager()
-
-# @pytest.fixture()
-# def plugin_registry(application):
-#     PluginRegistry._PluginRegistry__instance = None
-#     plugin_registry = PluginRegistry(application)
-#     plugin_registry._plugin_locations = [] # Clear pre-defined plugin locations
-#     return plugin_registry
-#
-# @pytest.fixture()
-# def upgrade_manager(application):
-#     VersionUpgradeManager._VersionUpgradeManager__instance = None
-#     upgrade_manager = VersionUpgradeManager(application)
-#     return upgrade_manager
-