fate: pass additional variables to test script

This passes the following additional variables to the test script:

- CMP  method to use comparing output with reference, default 'diff'
- REF  reference file, default 'tests/ref/fate/${test}'
- FUZZ maximum fuzz factor in comparison, meaning depends on CMP

Originally committed as revision 24133 to svn://svn.ffmpeg.org/ffmpeg/trunk
