pytest: Pretty print the memleak output in tests

It wasn't JSON formatted either so there was no nice pretty-printing
way. This jsonifies and pretty prints it.

Signed-off-by: Christian Decker <@cdecker>

tests/utils.py

@@ -1,3 +1,10 @@
+from bitcoin.rpc import RawProxy as BitcoinProxy
+from btcproxy import BitcoinRpcProxy
+from decimal import Decimal
+from ephemeral_port_reserve import reserve
+from lightning import LightningRpc
+
+import json
 import logging
 import os
 import random
@@ -9,12 +16,6 @@ import subprocess
 import threading
 import time
 
-from btcproxy import BitcoinRpcProxy
-from bitcoin.rpc import RawProxy as BitcoinProxy
-from decimal import Decimal
-from ephemeral_port_reserve import reserve
-from lightning import LightningRpc
-
 BITCOIND_CONFIG = {
     "regtest": 1,
     "rpcuser": "rpcuser",
@@ -864,7 +865,9 @@ class NodeFactory(object):
                     unexpected_fail = True
 
             if leaks is not None and len(leaks) != 0:
-                raise Exception("Node {} has memory leaks: {}"
-                                .format(self.nodes[i].daemon.lightning_dir, leaks))
+                raise Exception("Node {} has memory leaks: {}".format(
+                    self.nodes[i].daemon.lightning_dir,
+                    json.dumps(leaks, sort_keys=True, indent=4)
+                ))
 
         return not unexpected_fail