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_protocol.c

@@ -163,7 +163,7 @@ static void test_receive_str_truncated() {
   io_set_bwlimit(0);
   close(p[1]);
 
-  char* received = receive_str(0);
+  const char* received = receive_str(0);
   EXPECT_NULL(received);
 
   close(p[0]);