Create integration tests

[RedMser]

• Both the forked and default sample projects should contain a simple integration test. Load translations in different ways, translate a message with args, run the generator, ...
• Run this test either using a framework like Gut or via godot --headless --script test.gd
• Insert this into the CI pipeline, after the respective builds completed. (So after Windows Forked, run forked test. After Linux Default, run default test.)