[CI] Run unit test with non-editable installation (#845)

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`)

test/test_backend.py → test/torchaudio_unittest/backend_test.py

 import torchaudio
 
-from . import common_utils
+from torchaudio_unittest import common_utils
 
 
 class BackendSwitchMixin:

test/test_batch_consistency.py → test/torchaudio_unittest/batch_consistency_test.py

@@ -7,7 +7,7 @@ import torch
 import torchaudio
 import torchaudio.functional as F
 
-from . import common_utils
+from torchaudio_unittest import common_utils
 
 
 class TestFunctional(common_utils.TorchaudioTestCase):

test/test_compliance_kaldi.py → test/torchaudio_unittest/compliance_kaldi_test.py

@@ -6,7 +6,7 @@ import torch
 import torchaudio
 import torchaudio.compliance.kaldi as kaldi
 
-from . import common_utils
+from torchaudio_unittest import common_utils
 from .compliance import utils as compliance_utils
 
 

test/test_dataloader.py → test/torchaudio_unittest/dataloader_test.py

@@ -3,7 +3,7 @@ import unittest
 import torchaudio
 from torch.utils.data import Dataset, DataLoader
 
-from . import common_utils
+from torchaudio_unittest import common_utils
 
 
 class TORCHAUDIODS(Dataset):