Add txn_clone.py test

Does what the old txnmall.sh test did.

Creates an equivalent malleated clone and tests that SyncMetaData
syncs the accounting effects from the original transaction to the
confirmed clone.

qa/pull-tester/rpc-tests.sh

@@ -21,6 +21,8 @@ testScripts=(
     'mempool_resurrect_test.py'
     'txn_doublespend.py'
     'txn_doublespend.py --mineblock'
+    'txn_clone.py'
+    'txn_clone.py --mineblock'
     'getchaintips.py'
     'rest.py'
     'mempool_spendcoinbase.py'