Backend is not deterministic in test

[mthrok]

When I work on backend management logics, some tests that depends on specific I/O starts to fail.

In the following case, Kaldi compatibility test starts to fail when backend is somehow changed to soundfile.
(This also suggests that load_wav produces different results between sox backend and soundfile backend.)

https://app.circleci.com/pipelines/github/pytorch/audio/2114/workflows/e00835a3-c9bd-46d0-84ab-d9a705c4c968/jobs/54309/steps

To fix this, we need to make our test suite backend aware, and introduce a notation of default backend to be used unless separately specified. then have each test to specify/initialize the backend to be used.

[mthrok]

Another one from #698
https://app.circleci.com/pipelines/github/pytorch/audio/2220/workflows/7c9a50ec-2878-4a18-82c2-6e4ebcb102fc