This makes only tests actually using avconv depend on it.
The remaining tests already depend on what they need.
Signed-off-by: Mans Rullgard <mans@mansr.com>
instead of current "make output file of size less than ss".
Also use it to make MP3 tests more readable (using -fs xxx where xxx is
the requested output size, not something slightly lower).
Originally committed as revision 24884 to svn://svn.ffmpeg.org/ffmpeg/trunk
