pytest: fix btcproxy mock logic.

You're supposed to be able to hand mock_rpc either a function to call,
or a dict canned response.  We never did the latter, and the logic
was broken.

It was testing the key, not the value for whether it was a dict.  And
it could never have given a valid response anyway, since it wouldn't
know the id to use.  So assume it's a successful result.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/btcproxy.py

@@ -40,9 +40,13 @@ class BitcoinRpcProxy(object):
 
         # If we have set a mock for this method reply with that instead of
         # forwarding the request.
-        if method in self.mocks and type(method) == dict:
+        if method in self.mocks and type(self.mocks[method]) == dict:
+            ret = {}
+            ret['id'] = r['id']
+            ret['error'] = None
+            ret['result'] = self.mocks[method]
             self.mock_counts[method] += 1
-            return self.mocks[method]
+            return ret
         elif method in self.mocks and callable(self.mocks[method]):
             self.mock_counts[method] += 1
             return self.mocks[method](r)