tests/fate/api: fix fate-lavf-flv test

fate-lavf-flv is dependency of fate-api-seek, but the fate-api-seek is
setting CMP = null, which affects fate-lavf-flv. Reset CMP value to
expected by the flv test and use default diff compare.

Signed-off-by: Kacper Michajłow <kasper93@gmail.com>

tests/fate/api.mak

@@ -19,6 +19,7 @@ fate-api-h264-slice: CMD = run $(APITESTSDIR)/api-h264-slice-test$(EXESUF) 2 $(T
 FATE_API_LIBAVFORMAT-yes += $(if $(findstring fate-lavf-flv,$(FATE_LAVF_CONTAINER)),fate-api-seek)
 fate-api-seek: $(APITESTSDIR)/api-seek-test$(EXESUF) fate-lavf-flv
 fate-lavf-flv: KEEP_FILES ?= 1
+fate-lavf-flv: CMP =
 fate-api-seek: CMD = run $(APITESTSDIR)/api-seek-test$(EXESUF) $(TARGET_PATH)/tests/data/lavf/lavf.flv 0 720
 fate-api-seek: CMP = null