feat: clean up cached preprocessed audio after unittests (#386)
* feat: mixin to preprocess audio during unittests
* test: using setUp() instead of setUpClass()
* After Eric's help
* first working version
* fixup!
Using BasicTestCase for its contact & data_dir
* feat: declaring the class variables outsite a class method
* feat: renamed PreprocessedInputFixture to PreprocessedAudioFixture
* code review suggestions
