[GT4Py] Turn on new debug unit test for ALL_BACKENDS

[FlorianDeconinck]

While developing the debug backend, we added a tests/cartesian_tests/integration_tests/multi_feature_tests/test_debug_backend.py file for all unit tests.

Some of those tests should be elevated to ALL_BACKENDS. Other tests should be checked for debug markings.

Parent: #36

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• Trim the test_debug_backend.py for specifics tests
• Make sure coverage of features is complete for ALL_BACKENDS
• Make sure the backend=["debug"] tests are properly documented as to why it's debug only