Fix/expand gapless test

Fix a bug in the previous test (it was using the same file twice,
instead of using the both the positive/negative DC offset files), add
a similar test for m4a files, run at reduced volume, and automatically
adjust the test interval to the length of the files.

Change-Id: Ib421ec3d907daa52656b8dabe7fc4b6dabf401b6