fate/hevc: specify output pixel format explicitly

This allows running those tests with hwaccel.

tests/fate/hevc.mak

@@ -144,7 +144,7 @@ HEVC_SAMPLES_10BIT =            \
 
 define FATE_HEVC_TEST
 FATE_HEVC += fate-hevc-conformance-$(1)
-fate-hevc-conformance-$(1): CMD = framecrc -vsync 0 -i $(TARGET_SAMPLES)/hevc-conformance/$(1).bit
+fate-hevc-conformance-$(1): CMD = framecrc -vsync 0 -i $(TARGET_SAMPLES)/hevc-conformance/$(1).bit -pix_fmt yuv420p
 endef
 
 define FATE_HEVC_TEST_10BIT