Add a better fuzz test.

Attempts to verify all the properties defined in the security model,
while also being portable and able to run on many platforms.

tests/fuzztest/fuzz_syshdr.h

@@ -0,0 +1,15 @@
+/* This is just a wrapper in order to get our own malloc wrappers into nanopb core. */
+
+#define pb_realloc(ptr,size) counting_realloc(ptr,size)
+#define pb_free(ptr) counting_free(ptr)
+
+#ifdef PB_OLD_SYSHDR
+#include PB_OLD_SYSHDR
+#else
+#include <stdint.h>
+#include <stddef.h>
+#include <stdbool.h>
+#include <string.h>
+#endif
+
+#include <malloc_wrappers.h>