test: Skips scanner tags test when there's no mp3 support.

The file order returned by .find is not deterministic which made
testing the first scanner result fail when it happened to be an unsupported
media format (e.g. mp3).

tests/audio/test_scan.py

@@ -45,6 +45,9 @@ class ScannerTest(unittest.TestCase):
 
     def test_tags_is_set(self):
         self.scan(self.find('scanner/simple'))
+
+        self.check_if_missing_plugin()
+
         self.assert_(self.result.values()[0].tags)
 
     def test_errors_is_not_set(self):