Backend is not deterministic in test #715
Description
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.)
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.