tests/fate/avformat: Fix fate-lavf

The CMP variable seems to have been inherited from fate-api-seek which set it to null

the mxf reference needed a change due to c7e14a27Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b83c849e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

tests/fate/avformat.mak

@@ -61,6 +61,7 @@ FATE_LAVF += $(FATE_LAVF_PIXFMT-yes)
 
 $(FATE_LAVF): $(AREF) $(VREF)
 $(FATE_LAVF): CMD = lavftest
+$(FATE_LAVF): CMP =
 
 FATE_AVCONV += $(FATE_LAVF)
 fate-lavf:     $(FATE_LAVF)