refactor: move helper random_bytes to util library

Can be easily reviewed with `--color-moved=dimmed-zebra`.

test/functional/test_framework/util.py

@@ -12,6 +12,7 @@ import inspect
 import json
 import logging
 import os
+import random
 import re
 import time
 import unittest
@@ -286,6 +287,13 @@ def sha256sum_file(filename):
             d = f.read(4096)
     return h.digest()
 
+
+# TODO: Remove and use random.randbytes(n) instead, available in Python 3.9
+def random_bytes(n):
+    """Return a random bytes object of length n."""
+    return bytes(random.getrandbits(8) for i in range(n))
+
+
 # RPC/P2P connection constants and functions
 ############################################