refactor(direct message): create new dedicated test suite

Factored direct messaging related tests out of "ffa" test suite
and created a dedicated test suite for it.

Change-Id: I17b244d330ee634c4bd0c36a06160108de57953c
Signed-off-by: J-Alves <joao.alves@arm.com>
diff --git a/src/api.c b/src/api.c
index baa2d0e..abf2fae 100644
--- a/src/api.c
+++ b/src/api.c
@@ -2494,6 +2494,7 @@
 
 	if (!plat_ffa_is_direct_request_valid(current, sender_vm_id,
 					      receiver_vm_id)) {
+		dlog_verbose("Invalid direct message request.\n");
 		return ffa_error(FFA_INVALID_PARAMETERS);
 	}
 
@@ -2697,6 +2698,7 @@
 
 	if (!plat_ffa_is_direct_response_valid(current, sender_vm_id,
 					       receiver_vm_id)) {
+		dlog_verbose("Invalid direct response call.\n");
 		return ffa_error(FFA_INVALID_PARAMETERS);
 	}