Refactoring of audio_helper_tests.py

This makes a few changes to audio_helper_tests.py:

 - It removes some repeated code

 - There are more helpful error messages if the files are
   non-identical, telling you the filenames that differed.

 - If the files were different, the temporary files aren't
   removed, so you can examine the situation afterwards.

 - We also check that the type of the audio file (as
   reported by libmagic1 via python-magic) is the same as
   that expected.  This is the first check, and explains
   the critical differences that might stop the subsequent
   transcription in most cases.  (We may drop the file
   identical check later anyway.)
Factor out the check that the generated files are identical
Tests now check similar audio length rather than byte-by-byte identity

Subtly different versions of libmp3lame can cause different MP3s
to be generated for the same input, causing the tests that check
for the files being identical to fail.  Since we're already
checking that the type of the file (in a file(1) sense) is the
same, it makes sense to do a less strict check, so this commit
checks instead that the length of the audio of the file in
seconds is the same to 2 decimal places.