fix: resolve cppcheck warnings in new test files

- Remove unused ConsumerCtx struct in test_stress.c
- Add const qualifiers to variables only checked for NULL
  in test_metadata.c, test_protocol.c, test_robustness.c

tests/test_stress.c

@@ -19,15 +19,6 @@ typedef struct {
   int producer_id;
 } ProducerCtx;
 
-typedef struct {
-  Queue* q;
-  mtx_t* mutex;
-  cnd_t* cnd_empty;
-  cnd_t* cnd_full;
-  volatile bool* done;
-  int sum;
-} ConsumerCtx;
-
 typedef struct {
   Queue* q;
   mtx_t* mutex;