Test Data pseudo-mod

This mod is purely for loading data to be used by tests/cata_test. It is automatically loaded by tests/test_main.cpp, so any items, recipes, or other content defined in the mod will be available to everything in tests/.

The benefit of using this mod for test data is that it allows a clean separation of tests from in-game content. Instead of testing with content in the main data/json directory, functional tests can use TEST_DATA content to ensure a more stable and controllable set of example data.