[test] Move MY_RELAY to p2p.py

messages.py is for message and primitive data structures. Specifics
about the test framework's p2p implementation should be in p2p.py.

Also rename to P2P_VERSION_RELAY. Also rename msg_version.nRelay to
relay. In Bitcoin Core, this is referred to as fRelay, since it's a
bool, so this field has always been misnamed.

test/functional/test_framework/p2p.py

@@ -85,6 +85,8 @@ MIN_P2P_VERSION_SUPPORTED = 60001
 P2P_VERSION = 70016
 # The P2P user agent string that this test framework sends in its `version` message
 P2P_SUBVERSION = "/python-p2p-tester:0.0.3/"
+# Value for relay that this test framework sends in its `version` message
+P2P_VERSION_RELAY = 1
 
 MESSAGEMAP = {
     b"addr": msg_addr,
@@ -336,6 +338,7 @@ class P2PInterface(P2PConnection):
         vt = msg_version()
         vt.nVersion = P2P_VERSION
         vt.strSubVer = P2P_SUBVERSION
+        vt.relay = P2P_VERSION_RELAY
         vt.nServices = services
         vt.addrTo.ip = self.dstaddr
         vt.addrTo.port = self.dstport