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.
