fate: fix apng tests dependencies

Signed-off-by: James Almer <jamrial@gmail.com>

tests/fate/apng.mak

@@ -4,7 +4,7 @@ fate-apng-clock: CMD = framecrc -i $(TARGET_SAMPLES)/apng/clock.png
 FATE_APNG += fate-apng-osample
 fate-apng-osample: CMD = framecrc -i $(TARGET_SAMPLES)/apng/o_sample.png
 
-FATE_APNG-$(call DEMDEC, APNG, PNG) += $(FATE_APNG)
+FATE_APNG-$(call DEMDEC, APNG, APNG) += $(FATE_APNG)
 
-FATE_SAMPLES_FFMPEG += $(FATE_APNG)
+FATE_SAMPLES_FFMPEG += $(FATE_APNG-yes)
 fate-apng: $(FATE_APNG-yes)