test: use define for initializing ssl_message_queue struct

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/suites/test_suite_ssl.function b/tests/suites/test_suite_ssl.function
index ed78a96..6f9e544 100644
--- a/tests/suites/test_suite_ssl.function
+++ b/tests/suites/test_suite_ssl.function
@@ -12,6 +12,8 @@
 #include <constant_time_internal.h>
 #include <test/constant_flow.h>
 
+#define SSL_MESSAGE_QUEUE_INIT      { NULL, 0, 0, 0 }
+
 /* END_HEADER */
 
 /* BEGIN_DEPENDENCIES
@@ -459,7 +461,7 @@
 /* BEGIN_CASE */
 void ssl_message_queue_sanity()
 {
-    mbedtls_test_ssl_message_queue queue = {NULL, 0, 0, 0};
+    mbedtls_test_ssl_message_queue queue = SSL_MESSAGE_QUEUE_INIT;
 
     USE_PSA_INIT();
     /* Trying to push/pull to an empty queue */
@@ -481,7 +483,7 @@
 /* BEGIN_CASE */
 void ssl_message_queue_basic()
 {
-    mbedtls_test_ssl_message_queue queue = {NULL, 0, 0, 0};
+    mbedtls_test_ssl_message_queue queue = SSL_MESSAGE_QUEUE_INIT;
 
     USE_PSA_INIT();
     TEST_ASSERT(mbedtls_test_ssl_message_queue_setup(&queue, 3) == 0);
@@ -510,7 +512,7 @@
 /* BEGIN_CASE */
 void ssl_message_queue_overflow_underflow()
 {
-    mbedtls_test_ssl_message_queue queue = {NULL, 0, 0, 0};
+    mbedtls_test_ssl_message_queue queue = SSL_MESSAGE_QUEUE_INIT;
 
     USE_PSA_INIT();
     TEST_ASSERT(mbedtls_test_ssl_message_queue_setup(&queue, 3) == 0);
@@ -538,7 +540,7 @@
 /* BEGIN_CASE */
 void ssl_message_queue_interleaved()
 {
-    mbedtls_test_ssl_message_queue queue = {NULL, 0, 0, 0};
+    mbedtls_test_ssl_message_queue queue = SSL_MESSAGE_QUEUE_INIT;
 
     USE_PSA_INIT();
     TEST_ASSERT(mbedtls_test_ssl_message_queue_setup(&queue, 3) == 0);
@@ -574,7 +576,7 @@
 /* BEGIN_CASE */
 void ssl_message_queue_insufficient_buffer()
 {
-    mbedtls_test_ssl_message_queue queue = {NULL, 0, 0, 0};
+    mbedtls_test_ssl_message_queue queue = SSL_MESSAGE_QUEUE_INIT;
     size_t message_len = 10;
     size_t buffer_len = 5;