Exclude rand from debug log

Currently, debug.log is spammed with messages from random.cpp
when functional tests are run. These logs are not useful for
debugging, and decrease the signal to noise ratio of the logs.

test/functional/test_framework/test_node.py

@@ -102,6 +102,7 @@ class TestNode():
             "-debug",
             "-debugexclude=libevent",
             "-debugexclude=leveldb",
+            "-debugexclude=rand",
             "-uacomment=testnode%d" % i,
         ]
         if use_valgrind: