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/signmessages.py

@@ -2,13 +2,12 @@
 # Copyright (c) 2016 The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test RPC commands for signing and verifying messages."""
 
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import *
 
-
 class SignMessagesTest(BitcoinTestFramework):
-    """Tests RPC commands for signing and verifying messages."""
 
     def __init__(self):
         super().__init__()