Add Message Capture Test

Add a functional test for CaptureMessage.  This connects and then
disconnects a peer so that the handshake can be used to check if capture
is being done correctly.

Included in a docstring in the test is the following:

From the data file we'll only check the structure.

We won't care about things like:
- Deserializing the payload of the message
    - This is managed by the deserialize methods in
      test_framework.messages
- The order of the messages
    - There's no reason why we can't, say, change the order of the
      messages in the handshake
- Message Type
    - We can add new message types

We're ignoring these because they're simply too brittle to test here.

test/functional/test_runner.py

@@ -264,6 +264,7 @@ BASE_SCRIPTS = [
     'p2p_add_connections.py',
     'p2p_unrequested_blocks.py',
     'p2p_blockfilters.py',
+    'p2p_message_capture.py',
     'feature_includeconf.py',
     'feature_asmap.py',
     'mempool_unbroadcast.py',