GitLab

* Add test for CommonVoice dataset

* Migrate the existing tests for `bg_iterator` and `diskcache_iterator` to `test/datasets/utils_test.py`
Co-authored-by: Leon Gao <legao@linkedin.com>

Co-authored-by: lawrencechen <lawrencechen@devvm3189.vll0.facebook.com>

Co-authored-by: Edward Lu <edlu@fb.com>

* Update sample rate of libritts test
Co-authored-by: Ji Chen <jimchen90@devfair0160.h2.fair>

This makes it easier to test against an installed wheel, as the
torchaudio folder is no longer preferentially picked up when
you run a test module.

I had to move all tests in subfolders into the top level test
directory to make this work, since you can't access .. modules
without mucking around with sys.path (which I don't want to do.)

NB: this BREAKS the syntax where you can run a test by
saying `python -m test.test`.  Instead, do `python test/test.py`
or use the pytest runner.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>