test: move remaining random test util code from setup_common to random

and drop the util/random dependency on util/setup_common.

This improves code separation and avoids creating a circular dependency if
setup_common needs to call the util/random functions.

src/test/denialofservice_tests.cpp

@@ -15,6 +15,7 @@
 #include <script/standard.h>
 #include <serialize.h>
 #include <test/util/net.h>
+#include <test/util/random.h>
 #include <test/util/setup_common.h>
 #include <timedata.h>
 #include <util/string.h>