fate/api-tests: Tests that need samples should only run if SAMPLES is set

This change fixes a bug where a test that required a sample was being included
in the suite when SAMPLES was not set. It also improves the consistency of
variable names relating to the API tests.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

tests/fate/api.mak

@@ -4,12 +4,20 @@ fate-api-flac: CMD = run $(APITESTSDIR)/api-flac-test
 fate-api-flac: CMP = null
 fate-api-flac: REF = /dev/null
 
-FATE_API_SAMPLES-LIBAVFORMAT-$(call DEMDEC, H264, H264) += fate-api-h264
+FATE_API_SAMPLES_LIBAVFORMAT-$(call DEMDEC, H264, H264) += fate-api-h264
 fate-api-h264: $(APITESTSDIR)/api-h264-test$(EXESUF)
 fate-api-h264: CMD = run $(APITESTSDIR)/api-h264-test $(TARGET_SAMPLES)/h264-conformance/SVA_NL2_E.264
 
-FATE-$(CONFIG_AVCODEC) += $(FATE_API_LIBAVCODEC-yes)
-FATE_API_SAMPLES-$(CONFIG_AVFORMAT) += $(FATE_API_SAMPLES-LIBAVFORMAT-yes)
-FATE_SAMPLES_AVCONV += $(FATE_API_SAMPLES-yes)
+FATE_API_SAMPLES-$(CONFIG_AVFORMAT) += $(FATE_API_SAMPLES_LIBAVFORMAT-yes)
 
-fate-api: $(FATE_API_LIBAVCODEC-yes) $(FATE_API_SAMPLES-yes)
+ifdef SAMPLES
+    FATE_API_SAMPLES += $(FATE_API_SAMPLES-yes)
+endif
+
+FATE_API-$(CONFIG_AVCODEC) += $(FATE_API_LIBAVCODEC-yes)
+FATE_API-$(CONFIG_AVFORMAT) += $(FATE_API_LIBAVFORMAT-yes)
+FATE_API = $(FATE_API-yes)
+
+FATE-yes += $(FATE_API) $(FATE_API_SAMPLES)
+
+fate-api: $(FATE_API) $(FATE_API_SAMPLES)