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Currently our test suites automatically/silently skip tests on C++ extension
if it is not available. This is nice in local env, but in CI these tests should be
enforced and reported as failure if C++ extension is not available.

This PR adds switch for making tests fail if C++ extension is not available,
and make CI for *nix fail if that's the case.

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.

buck gets confused with utility module name with `test_` prefix.

* Get rid of sine wave files and whitenoise files
* Refactor integer encoding
* Relax rtol from 1e-8 to 1e-7 for compliance kaldi
* relax waveform multi channel resample atol to 1e-7 from 1e-8
* relax tolerance for length consistency for speed effect
Co-authored-by: moto <855818+mthrok@users.noreply.github.com>

* Add sox_utils module

* Make init/shutdown thread safe

* Add sox effects implementation

* Add test for sox effects

* Update docstrings and add examples

This PR replaces `torchaudio.sox_effects.SoxEffectsChain` in `test_sox_compatibility` with bare `sox` command.

The parity of `torchaudio.sox_effects.SoxEffectsChain` against `sox` command is not tested and it has known issues https://github.com/pytorch/audio/issues/771, therefore it is not appropriate to use this class for testing other functions.

Part of #764

 - Replace `whitenoise.wav` with on-the-fly data generation
 - Replace `torchaudio.load` with `common_utils.load_wav`
 - Replace `steam-train-whistle-daniel_simon.mp3` with `.wav`