test: [WFE] functional test of no matching words

To check we do not match any words that are not present in a media file with
speech, we use the English-language Harvard sentences for audio, and compare to
the classic French pangram: "Portez ce vieux whisky au juge blond qui fume" [1]
and ensure no Features are produced

[1]: "Take this old whisky to the blond judge who is smoking"

test/test_feature_extractors_functional.py

@@ -254,5 +254,29 @@ class TestWordFEFunctional(FEFunctionalTest):
         with self.assertRaises(RuntimeError):
             testfe.run()
 
+    def test_audio_word_detection_harvard_no_matching_words(self):
+        """Test audio word detection feature extractor
+           Uses:
+             - sample-manual-audio-harvardsentences-video-colours.mp4
+               :: Harvard sentences (list 1) up to item 1.8 ("The birch canoe... The hogs were fed")
+                  plus the sentence: "Portez ce vieux whisky au juge blond qui fume"
+                  -- pass if no features extracted
+        """
+
+        SAMPLE_VIDEO = f"{self.SAMPLE_DIR}/sample-manual-audio-harvardsentences-video-colours.mp4"
+        DETECT_WORDS = "Portez ce vieux whisky au juge blond qui fume".split()
+
+        # create mock source with the video
+        source = mocks.MockSource(path=SAMPLE_VIDEO)
+
+        # create the feature extractor
+        testfe = extractors.WordFeatureExtractor(input_files=[source])
+        testfe.setup(words=DETECT_WORDS)
+        testfe.run()
+        testfe.teardown()
+
+        # ensure no Features:
+        self.assertEqual(len(testfe.features), 0)
+
 if __name__ == "__main__":
     unittest.main()