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We have been running unit test with editable installation. (i.e. `python setup.py develop`), with which we missed issues like #842. 

This CC makes installation in CI non-editable, and change test directory structure so that the source code will not shadow the installed version of `torchaudio`. With simple `pytest test`, `pytest` modifies `sys.path` and prepend checked out repository, which shadows the installed version.

To remedy this, the whole test suites has been moved from `./test` to `./test/torchaudio_unittest`. This adds nice module structure to our test code and we can do absolute import in each test module, which makes it possible again to run test with `python -m unittest torchaudio_unittest/XXX.py`

This change does not affect the regular development process (`python setup.py develop` && `pytest test`)

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>