Fix walletbackup.py after directory restructuring

Also add walletbackup.py to rpc-tests.sh

qa/pull-tester/rpc-tests.sh

@@ -31,6 +31,7 @@ testScripts=(
     'proxy_test.py'
     'merkle_blocks.py'
     'signrawtransactions.py'
+    'walletbackup.py'
 );
 testScriptsExt=(
     'bipdersig-p2p.py'

qa/rpc-tests/walletbackup.py

@@ -33,8 +33,8 @@ Shutdown again, restore using importwallet,
 and confirm again balances are correct.
 """
 
-from test_framework import BitcoinTestFramework
-from util import *
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import *
 from random import randint
 import logging
 logging.basicConfig(format='%(levelname)s:%(message)s', level=logging.INFO)