refactor: use <cstdlib> over stdlib.h

We currently use both. Consolidate on the former.

src/test/blockchain_tests.cpp

@@ -4,13 +4,13 @@
 
 #include <boost/test/unit_test.hpp>
 
-#include <stdlib.h>
-
 #include <chain.h>
 #include <rpc/blockchain.h>
 #include <test/util/setup_common.h>
 #include <util/string.h>
 
+#include <cstdlib>
+
 /* Equality between doubles is imprecise. Comparison should be done
  * with a small threshold of tolerance, rather than exact equality.
  */