Migrate Test Suite

• Adapt the entire pystencils test suite to the changes introduced with version 2.0.
• Clean up the tests and separate them into subfolders.
• Mark still-failing tests as xfail
• Reintroduce tests-and-coverage CI task to run the full test suite
• Temporarily reduce coverage threshold to 80%
• Remove tests relating only to the old code generation backend

Don't be too afraid of the number of changes; most of them are just file moves.