Add tests for FFA function FFA_SPM_ID_GET

Also add the ability for ffa_features_test to only use the expected
return if the minimum FF-A version for the feature is met.

Signed-off-by: Daniel Boulby <daniel.boulby@arm.com>
Change-Id: I276500c666365c563a117aa7b250b18f51dcace7
diff --git a/spm/cactus/cactus_main.c b/spm/cactus/cactus_main.c
index cc2072c..73606bd 100644
--- a/spm/cactus/cactus_main.c
+++ b/spm/cactus/cactus_main.c
@@ -182,7 +182,7 @@
 
 	/* Get current FFA id */
 	smc_ret_values ffa_id_ret = ffa_id_get();
-	ffa_id_t ffa_id = (ffa_id_t)(ffa_id_ret.ret2 & 0xffff);
+	ffa_id_t ffa_id = ffa_endpoint_id(ffa_id_ret);
 	if (ffa_func_id(ffa_id_ret) != FFA_SUCCESS_SMC32) {
 		ERROR("FFA_ID_GET failed.\n");
 		panic();