Fix some unit tests.

Add fixtures to the conftest file and clean-up the code a bit.

Contributes to CURA-5628

tests/conftest.py

@@ -0,0 +1,38 @@
+# Copyright (c) 2018 Ultimaker B.V.
+# Cura is released under the terms of the LGPLv3 or higher.
+
+# 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 cura.CuraApplication import CuraApplication
+from cura.MachineActionManager import MachineActionManager
+
+# Create a CuraApplication object that will be shared among all tests. It needs to be initialized.
+# Since we need to use it more that once, we create the application the first time and use its instance afterwards.
+@pytest.fixture()
+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
+