Move random test util code from setup_common to random

as many of the unit tests don't use this code

src/test/serfloat_tests.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <hash.h>
+#include <test/util/random.h>
 #include <test/util/setup_common.h>
 #include <util/serfloat.h>
 #include <serialize.h>