Add constant background test to Core/GUI/Python-StandardTest

Core/StandardSamples/SimulationFactory.cpp

Core/StandardSamples/StandardSimulations.cpp

Core/StandardSamples/StandardSimulations.h

GUI/coregui/Models/TransformFromDomain.cpp

GUI/coregui/Models/TransformFromDomain.h

Tests/Functional/Core/CoreStandardTest/CMakeLists.txt

Tests/Functional/GUI/GUIStandardTest/CMakeLists.txt

Tests/Functional/Python/PyStandard/CMakeLists.txt

Tests/Functional/TestMachinery/StandardTestCatalogue.cpp

Tests/ReferenceData/StandardSuite/ConstantBackground.int.gz