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Now that most of our test cases are backend aware/agnostic,
the use of BACKENDS and BACKENDS_MP3 is anti-pattern.

This change moves these definitions to the legacy test module `test_io`.

If a new backend is added in future, that backend should have separate test suite,
(like the ones in `test/sox_io_backend`) so instead of dynamically listing available
backend in `test_io`, the changed code only checks "sox" backend and "soundfile" backend.