qa: add a test to detect leaky p2p messages

This is certainly not exhaustive, but it's better than nothing. Adds checks
for:

- Any message received before sending a version
- Any message received other than version/reject before sending a verack

It also tries to goad the remote into sending a pong, address, or block
announcement.

qa/pull-tester/rpc-tests.py

@@ -154,6 +154,7 @@ testScripts = [
     'bumpfee.py',
     'rpcnamedargs.py',
     'listsinceblock.py',
+    'p2p-leaktests.py',
 ]
 if ENABLE_ZMQ:
     testScripts.append('zmq_test.py')