[test] Move MY_SUBVERSION to p2p.py

The messages.py module should contain code and helpers for
[de]serializing p2p messages. Specific usage of those messages should
be in p2p.py. Therefore move MY_SUBVERSION to p2p.py.

Also rename to P2P_SUBVERSION.

test/functional/test_framework/p2p.py

@@ -83,6 +83,8 @@ MIN_P2P_VERSION_SUPPORTED = 60001
 # The P2P version that this test framework implements and sends in its `version` message
 # Version 70016 supports wtxid relay
 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/"
 
 MESSAGEMAP = {
     b"addr": msg_addr,
@@ -333,6 +335,7 @@ class P2PInterface(P2PConnection):
         # Send a version msg
         vt = msg_version()
         vt.nVersion = P2P_VERSION
+        vt.strSubVer = P2P_SUBVERSION
         vt.nServices = services
         vt.addrTo.ip = self.dstaddr
         vt.addrTo.port = self.dstport