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/malloc_wrappers.h

@@ -0,0 +1,7 @@
+#include <stdlib.h>
+
+void* malloc_with_check(size_t size);
+void free_with_check(void *mem);
+void* counting_realloc(void *ptr, size_t size);
+void counting_free(void *ptr);
+size_t get_alloc_count();