Since insertion of aud landed, we need to change some testcases
accroding to the change.
Note that counting frames are changed in parser.c,
due to generated frames, AUD.
https://bugzilla.gnome.org/show_bug.cgi?id=736213
Allow context callbacks to perform custom buffer verification
and custom element setup. Also move some static tracking variables
into helper struct and expose the latter struct for use by unit test
(callbacks).
