Fix some unit tests.

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

Contributes to CURA-5628

tests/Settings/conftest.py

@@ -0,0 +1,19 @@
+# Copyright (c) 2018 Ultimaker B.V.
+# Uranium 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 settings tests.
+
+import pytest
+import copy
+
+from UM.Settings.DefinitionContainer import DefinitionContainer #To provide definition containers in the registry fixtures.
+
+# Returns the CuraContainerRegistry instance with some empty containers.
+@pytest.fixture()
+def container_registry(application):
+    return application.getContainerRegistry()
+
+# Gives an arbitrary definition container.
+@pytest.fixture()
+def definition_container():
+    return DefinitionContainer(container_id = "Test Definition")