Make "-fs ss" mean "make output file of size equals or less than ss"

Make "-fs ss" mean "make output file of size equals or less than ss"
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