Make TestCases backend-aware (#719)

* Make tests backend aware by introducing TorchaudioTestCase and reset backend for each TestCase.

* Set backends for the test cases that require specific backend.

test/torchscript_consistency_impl.py

@@ -616,5 +616,6 @@ class Transforms(common_utils.TestBaseMixin):
 
     def test_Vad(self):
         filepath = common_utils.get_asset_path("vad-go-mono-32000.wav")
+        common_utils.set_audio_backend('default')
         waveform, sample_rate = torchaudio.load(filepath)
         self._assert_consistency(T.Vad(sample_rate=sample_rate), waveform)