Revise VGGish inference pipeline test (#3544)

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/3544

Revises VGGish inference pipeline test to support internal testing.

Reviewed By: mthrok

Differential Revision: D48058409

fbshipit-source-id: 045140a0e9d50128d32ef6510bdb2f642a365c83

test/integration_tests/prototype/vggish_pipeline_test.py

+import unittest
+
 import torchaudio
 from torchaudio.prototype.pipelines import VGGISH
 
 
-def test_vggish():
+class VGGishPipelineTest(unittest.TestCase):
+    def test_vggish(self):
         input_sr = VGGISH.sample_rate
         input_proc = VGGISH.get_input_processor()
         model = VGGISH.get_model()