Fix docstrings in qa tests

This commit fixes the module-level docstrings for the tests and helper
modules in qa. Many of these tests were uncommented previously - this
commit ensures that every test case has at least a minimum level of
commenting.

qa/rpc-tests/test_framework/bignum.py

@@ -1,15 +1,11 @@
 #!/usr/bin/env python3
 #
-# bignum.py
-#
-# This file is copied from python-bitcoinlib.
-#
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
-#
-
-"""Bignum routines"""
+"""Big number routines.
 
+This file is copied from python-bitcoinlib.
+"""
 
 import struct